If you just need a "spare tire wheel" and do not care if it matches, there are some non-Opel wheels and space-saver spares likely available from your local auto-recycler yard that will work well. Saves a large shipping cost from OU in California.
Some folks will suggest ANY 4x100 PCD wheel, such as a mid '80's to late '90's vintage Honda Civic, Accord or Prelude, but the factory wheels may have a centre hole that is too small (56.1 mm, needs to be 57.1 mm), and that will NOT fit over the Opel's front bearing hub. And too large an offset (ET's are typically +38 to 42 mm, should be +25 mm). But they are worth checking.
Early to late '90's VW Golf's and Passat's had the correct PCD (stud spacing, 4x100 mm) and hub size (57.1 mm), although the offset (ET) was +36 to 40 mm, so the tire centre will be somewhat inboard. But a thin'ish (185 mm or smaller) tire will work in either case.
I found a very good solution by using the space-saver wheel/tire from the mid-90's VW Passat. The wheel is 15" (versus 14" for the Golf), and the diameter is very close to the Opel's size. The tire is 2/3 the width of a stock Opel tire, which frees up some room in the spare tire compartment, but the factory wire hold-down may not properly clamp the tire in place. My solution was to install a proper centre-bolt hold-down from under the spare tire shelf. That allows the spare to be located a bit closer into the rear corner of the spare tire shelf, and the wheel centre space (with the outside facing down) provides even more storage room.
HTH