Hmmm... Give avsim.com a try. Also, I think simviation.com has a fairly sizable FS98 collection. These two sites are where I go to find all my FS
aircraft. But I can't tell you for sure if they'll have a lot of FS98 a/c, because it's been so long since I've looked for them. But if I remember correctly, there are still a few sites that have FS95 and FS5.0 aircraft, so you ought to be able to find some good stuff on FS98.
One thing though, if you are looking for general aviation aircraft, the FS98 stuff probably won't be as good as the default aircraft. But if you are looking for airliners, there is (was) a lot of good stuff for 98, and it was much better than the default.