If you search on alibaba, you can often find them cheap.
With the ones shown below you can buy ten for $13.50 plus shipping, that's $1.35 each. With shipping, it might be under $2.50 each, check for yourself. These are 11T. I have a modern reproduction of a derailleur similar to yours, with 10T jockey wheels. With my derailleur, I believe I can run 11T, but no larger. Have a good look at your derailleur, and see if 11T will work.
There are also 10T jockey wheels on alibaba, but they are less common. Search for yourself.
You can also buy new derailleurs.
Where I am I can buy derailleurs real cheap from the local shops. But I have thought about, after I return to Australia, maybe buy 50 jockey wheels. That would be more than you are likely to use in your lifetime. You should be able to get 50 for less than $50. You could then give some away to your cycling friends, or people you meet with very worn jockey wheels, as they cost next to nothing.
Chinese can scam you, but for a purchase of $25, I would not be overly concerned.
