Be careful; be very careful. There are Vst's galore out there, and very soon the hard disk is full, and you can't find the one you want in Reaper (or any other DAW...), it's so choked up. How do I know..? Ha..!
Decide what genre you're after (Instruments..? Fx..? Treatment..? Drums..? The list goes on...). OK, instruments. Modern band stuff (Guitars, bass, keys, brass...)..? Orchestral (No, I can't list here; there's really too much...)..? Ethnic flutes, steel drums, didgeridoos..? No..? OK, Fx. Dirt boxes..? Delay, reverb, phase, flange, whatever... Treatment..? Compressors, mastering tools, de-noisers...
Tell us what interests you the most, in the first instance, and we can go from there. Be aware: this way, madness lies. Be careful; be very careful.
Hope this helps.