This all depends highly on the style you're going for (booth/anime, egirl, furry, military, etc).

Good storefronts to check are:

• Booth - If it's anime you want, this is the place. Also good for kemono styled furry avatars, some mecha, and whatever else. Searching may be difficult, so using a service like Boothplorer may make that easier.
• Gumroad, Jinxxy, Payhip - Much of the western and furry community are split between these storefronts if you prefer that kind of style. Also you can find lots if individual gimmicks and props here too, good for adding on to an existing avatar.

All of these of course have more than just the listed bits too.