The stem you need is almost certainly to fit a 1" threaded steerer. Most older non-American (and some American) bikes will take the same size. Make sure the stem is long enough to get the bars where you want them. There is more variation in the stem-to-handlebar size, but if you find a bar-stem combo that you like, you'll be set. When you are shopping the used bikes, you could take a caliper set to the stem diameter you want to replace. You might find a cheap caliper that would work at a hardware store, or try a large crescent wrench for sizing. If the size is different, it will be different by 1/8 inch, so high accuracy is not necessary.