My mom lost the back post to one of her earrings-I believe they are a screw-back post (is that the correct term?) Where is the best place to find and buy a replacement or replacement pair?
You didn't mention whether it is a nice pair of earrings (gold/diamonds) or a fashion pair.
For earrings that are not terribly expensive, I have had great luck finding backs online by doing a google search. There are a lot of companies that supply products for people that make and sell jewelry. For example, those clear plastic "stops" that you can put on the back of fishhook earrings. I bought a pack at Claire's once and got maybe 20. For the same price at a online supplier I was able to get 150.