My favorite places for buying (I've skipped free sources) type are —

Collectives

FontFont

FontShop

MyFonts

Font Bureau

Commercial Type

All these sites offer work from multiple foundries and sometimes their own. Commercial Type and Font Bureau represent the more exclusive and storied typefaces from many famous contemporaries in the field of type design today. While the others have such variety from the big guys and the little ones, that you'll find a great font. Sites like MyFonts are really good at comparing classic typefaces like Helvetica, Futura, or Garamond to help narrow down the cut that works for you.

Individual Foundries

Darden

Okay Type

House Industries

Sudtipos

Darden, Okay, House, and Sudtipos are great places to find display and specility typefaces that are very extensive and well crafted. Don't miss the cursive scripts from each of them. The ligatures and extended features are very exciting.

Emigre

Their Mrs. Eaves font has held up through fad and fashion very well.

Storm Type

This Czech foundry is a little under the radar, but their recuts of Baskerville and Walbaum are exquisite. They've done a fantastic job modernizing classic types like those and others. They also have a few in-house fonts they've designed based on Czech morphologies that I'm dying to use on a future project.

The Foundry

Foundry's serif fonts are some of the best, and would be very well suited in editorial or print work.

Frere-Jones

After the messy split with Hoefler, Tobias launched his own foundry and offers one font currently. You'll also find some of his pre-Hoefler work on Font Bureau.

Playtype

Based out of Copenhagen, these guys have fun and quirky type great for branding. I've used them on a few projects.

Grilli Type

Nouvelle Noire

These Swiss foundries have very clean and sophisticated fonts with flair and whimsy built at a very subtle (or not so subtle) level. I haven't used any of their work yet, but I can see the potential.

Milieu Groteseque

I think these guys are underrated compared to Klim. I've found their type to be very well suited to pratical (even UI) as well as branding use.

Hoefler & Co.

The fonts here are expertly crafted and you can almost never go wrong using them. Their attention to detail and the craft of creating type is very evident. They're a powerhouse for good fonts, as they cover every major category well.

Klim

The typefaces here are great. They're boutique but very popular, like your favorite Indie brand.