I just purchased a used D90. I had a tripod but I just found that the D90 didn't have the tripod mount (the adapter to mount the D90 to the tripod). Is there anywhere online I can buy that?
This has nothing to do with the camera. All cameras have the same thread which is used to attach it to a support like a tripod.
What you are looking for is a quick-release plate and that is specific to the tripod head you use. If you have the exact model, you have to look it up and find out the part number for the plate. That is the one to order. For example my head is a Manfrotto 057RC4 and it uses a 410PL plate.
Depending on your tripod, availability may vary or it may be a special order. If you use one of the major brands like Manfrotto, Gitzo, Velbon, Giotto, etc then B&H Photo or Adorama stock a good number of them.
If you do not have a model number or part number, then the best is to bring to a store, ideally the one where you bought the head (or tripod if it came with a head) and they will tell you the part number and order it at your request.