By gaskets do you mean the seal between window frame and sidewall? Most that I have seen are just strips of caulking tape, usually a butyl-based rubber sealant. You buy a roll and stick it on when re-installing the window. Mobile home parts places always have it on hand, but so do many home centers and hardware stores.
If yours has a rubber seal, here's a source for a variety of types:
http://www.all-rite.com/rubber-seals-gaskets-c-38.html
http://www.rvecafe.com/win.html