I may try some different pickups in a guitar, but I don't know which brand or model. I can listen to Youtube videos or sound files all I want, but none of those will tell me what the pickups will sound like on my guitar.

I bought some Klein Vintage 59 pickups for my Strat a couple of years ago and lucked out. They sounded great. The problem is that Kleins and other brands are expensive. It would be easy to burn through money trying different models.

Is there a good place to find used pickups?
not really. some guitr shops that do a lot of repairs might have the occasional used pick up laying around. places like craigslist also have used pickups but i rarely see anthing that is good mostly hey i replaced my epi LP pups so i'm selling the originals. ebay may have them as well.
eBay and Craigslist.
Ibanez RG1570 Prestige
Jackson Kelly KE3 - MIJ (Distortion/Jazz)
Jackson DKMGT Dinky (EMG 81/85)
ESP E-II Eclipse Custom (JB/'59)
ESP LTD EC-1001FR (EMG 81/60)
Fender MIM Strat

Mesa/Boogie Dual Rectifier Roadster 212
Laney IronHeart IRT-Studio
Peavey Vypyr 30
Peavey ReValver Amp Sims
TOOOO many T.C. Electronic Pedals. . .
ebay, craigslist, reverb, SSO marketplace, Music Gear Exchange on Facebook, any other guitar forum marketplace.
Gibson RD Silverburst w/ Lace Dissonant Aggressors (SOLD)
Electra Omega Prime Ceruse
Fender Franken-Jag Bass

Amps and the like:
Laney VH100R
Seismic Luke 2x12
Dunlop 105Q Wah
Gojira FX 808
Line 6 M9