Found, Yes..
Often.. No. Any sheetemetal you find in the US for the GT is cut off of donor cars. They do have oversized replacement panels in europe. But the cost is extreme and the quality is marginal. Which is probably why Opel GT Source doesnt carry those parts.
As for found.. almost any part anyone needs is sitting in the garage or attic of someone on this forum (Or on the classicopel mailing list) All you have to do is ask. Some parts are rarer than others, and finding someone willing to part with a part they have is another subject as well.
As for your need of rear quarters, and since you are new to the forum (and possbily Opels), I will try to give a quick unbiased look at the Opel parts sources. Since there are a lot of mixed messages about a few of the companies on the forum.
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Opel GT Source -
http://www.opelgtsource.com
They have a huge selection of NOS (New Old Stock) parts, new, reproduction and some used parts. Thier engine, brake, and misc mechanical parts selection is really complete, they have most parts for the 1.1L and 1.9L, GT, Manta and Kaddett. Althoguh they primarily support the GT, they do carry a few Kadett and Manta specific trim and suspension parts. They have invested a lot of money and time into reproduction of GT trim and mechanical parts, they have reproduced all the lenses, most the rubber, etc... not only are the reproductions affordable, they are of some of the best quality I have seen in non-OEM reproduction parts for any make or model.
Pros: Quick, Friendly, Easy communication, lot of new and reproduction parts that are hard or impossible to get anywhere else.
Cons: No sheetmetal, few used parts (Mainly only parts that are not available any other way, window regulators, etc.. )
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OpelUSA -
http://www.USAOPEL.COM
Jim at OpelUSA is also easy to get in touch with, and does have a decent selection of primarily used and some NOS parts. Though not having been to his place I can't say exactly what he has in stock at any given time. His prices can seem to be high at times, and his knowledge Opels outside of the GT is not as deep as some other Opel businesses, but if you know what you want he can usually help you.
Pros: Easy to get in touch and work with, quality of parts is as he describes on the phone.
Cons: Limited Opel knowledge on non-GTs, Pricing a bit high on some parts
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Opels Unlimited - Opelsunl.com
OU has undoubtedly the largest selection of old sheetmetal, NOS, and other Opel parts for all Opels imported in the US. I have been told they have even started producing fiberglass rear quarter panels, hoods and other parts. The biggest problem with Opels Unlimited is that they are a pain to get ahold of, and then when you do, delivery times can sometimes take a while. What I have seen from them in the past is if the part is a fairly common part, or easy for Todd to get his hands on the quicker you get the part. But if you need him to cut a car for a specific piece you need, that can take a lot of calling, bugging and waiting.
Speaking of specifics, be sure to be clear what you want in the part you are buying. Case in point with rear quarters... don't ask him for a nice rear quarter with no rust and expect it perfect. It may have dents, and patches of bondo in it.. they are used parts afterall. If you want a PERFECT rear quarter, no dents, no bondo and no rust, tell him that... you have to be VERY specific... Mainly because, as we all are, he is dealing with a limited number of cars and parts left. For the sake of argument say a quarter panel from him may run $50-$500. If you only want to spend $150 and have a rust free fender he will get you that.. (With the bondo and dents), if you want a PERFECT one, thats going to run $500.
That said, once you get to know Todd, he truly is not a bad guy. His biggest problem is a lack of willingness to learn the computer, get up before 2PM PST, and hire good help. Speaking of which, best time to reach him is between 5pm and 10pm PST. Those are as close as you will ever get to what could be called "normal business hours".
Pros: Huge Inventory
Con: Hard to get in touch with, quality of parts varies widely at times (If you are very specific its usually not a issue).
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Goin Manta -
http://www.OpelManta.com
Hmm... review myself... I guess I can do that.
Simply, I can be slower that a turd in below zero temperature. Mainly due to the fact I carry little to no stock. I import almost everything. Secondly, I am VERY specific, unlike Gil at Opel GT Source, who supports the GT, but helps the Manta and Kadett people, I don't.
OK well I do, all but the GT people that is, I just send you guys to the other sources. I also do not deal in engine, brakes, or any of the other mechanical parts, there are sources for those parts already. I specialize in Opel Manta/Ascona A Sheetmetal, trim and rubber parts. I also will hunt down parts for gray market Opels, Kadetts, etc..
Because of my low volume, and volatile Euro exchange rate my prices are ridiculous. Even to me... recently I have been selling the imported parts at cost (or as in the case of the last order, once imports and customs was done with me, I lost money on everything).
I do it as a hobby, to support my hobby. About 40% of the last order from Germany I did was for my personal needs.
Pros: Easy to get in touch with, helpful, can get the impossible to find parts for Mantas, and gray market cars.
Cons: Slow and too expensive.
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Hope that helps.
Charles