There really is no good "one stop shop" for boards. Buckling Springs you'll want to try your hand at eBay or
Clicky Keyboards - for Model M reps you'll want
Unicomp, For Cherry boards, there's a variety of 'point of sales' websites. For Filco's (if you're in the United States), go to Elitekeyboards. For HHKB and Topre's you'll have to make do with
GeekStuff4U or
stlensbar (at least until elitekeyboards starts stocking capacitives AHEM). Really they're kind of all over the place. You might want to sign up to Newegg's newsletter, About once every 3 months I get a promo in the mail that makes ABS M1 (black alps) $45 with free shipping. Elitekeyboards will be stocking a very small number of blue cherry Filco's so keep your eyes open for that (sign up to their newsletter).
In the
Geekhack Wiki there's a few threads that provide links to US, UK and JPA shops for buying keyboards of all types of switches, but they're all different places.