Ive been to almost every parts place known to man (This one anyway)Everybody suggests OTGS there prieces are outrageous. They all have a listing will take your order then 3 days later tell you no dice.
While I agree that sometimes OGTS prices are higher than some other internet auto shops, I always buy my parts from Gil & Dennis at OGTS. They have NEVER steered me wrong, always stand behind their products & keep making a variety of parts that keep our beloved Opels in good repair.
Unless things have changed in the last week or so, no one has new or rebuilt master cylinders for GTs. Best bet is get a rebuild kit from OGTS and rebuild it yourself. It should only take about an hour or so to remove, rebuild, reinstall and bleed. But before you reinstall, bench bleed the MC or it will take a long time to get it operational.