This is probably much more useful for 3D newcomers, but I thought that I should post where I buy paid content most often and why - essentially, this is my Top 3 list:
3.) Renderotica (link withheld due to DA TOS)
While I do not focus on explicit imagery or super-heavy BDSM, Renderotica is still quite useful for content. For example, while many poses offered at Renderotica are intended for sexual and/or BDSM usage, many can be adapted for non-sexual, non-kinky purposes (such as a lesbian pose set being used, with some minor modifications, to show best friends sharing a yuri moment together). Various morphs and characters are also available, as are settings and props which are not inherently sexual or kinky in nature. The Renderotica weekly e-mail and occasional Tumblr posts are good ways to learn about what is new on the site. There is also a points system available so that buying content gains points which can be used to reduce the price of a future purchase.
2.) Renderosity (www.renderosity.com
While I do not buy nearly as much content here now as I once did, Renderosity is still a site I visit almost daily to see what is new. Perhaps the biggest benefit of Renderosity versus the other sites mentioned here is that new content is added throughout the day every day. The Prime membership makes the site even better, with a discount on many items at any given time, plus Prime-only coupons sent via e-mail for special sales or steeper discounts on the regular sales underway. For those who use Dawn, Dusk, or Luna, Renderosity offers a good amount of content. The site is also a great place to find backgrounds, brushes, and other items useful in 3D postwork (primarily for Photoshop, but some of it also works in GIMP). One other benefit is that some of the content on Renderotica is also on Renderosity, for those who want to find certain content without being subjected to a lot of in-your-face adult content. Renderosity is also still very much supporting VAGS4 content, with new VAGS4 characters and clothing appearing on a somewhat-regular basis even though the 3D world is now up to Genesis 3 Female/Male.
1.) Daz (www.daz3d.com
This is very much a no-brainer for me since I use Daz Studio. Other than Dawn, all of my character bases have come from Daz. Daz also has an extremely deep back catalogue (going back at least to Victoria 2), and with a PC+ membership, much of that content is almost dirt cheap even when it is not on sale. A PC+ membership can be fairly expensive when not on sale, but it truly does pay for itself in terms of a monthly coupon for free content, another monthly coupon for $6 off an $18 purchase, and monthly and weekly freebies. New content is only added once per day (shortly after Midnight Mountain Time), but the offers on new content generally last for several days; even better, PC+ members get 30% off all new content. Also, much of the old RuntimeDNA content is now available at Daz (due to last year's acquisition).
A few other notes:
A.) All three sites mentioned above also have galleries, which can be useful to see how content sold at those sites are used in a variety of situations. Renderosity has the best content-to-image system: On a product page, clicking Gallery will show images which specifically use that content; on an image page, content used is also shown with thumbnail info and pricing.
B.) Many of the content creators selling at these sites are also at least semi-active on DA, so it is worth following them here to learn about their upcoming offerings and learn about their sales on the various sites.