Good question. I found this site:
Jeff's Photoshop Tips and Tricks - Filters
but plugin/filter sites that specifically mention
PS 5.5 are not very common - probably because (as you observed) it's really old technology.
According to their websites,
Harry's Filters,
Plugin Galaxy and
Filters Unlimited all work in
PS 3 and above:
All I can suggest for the rest is that most plugin makers will let you download a free demo version of their product which you can use to find out if it is compatible with your version of PS.