The really basic boring ones are Booth avatars, and you customize them with other assets that you acquire. The easiest assets being Booth assets, but you can use Western assets as well. It's not a common practice to sell customized Booth avatars, and you will certainly find none on Booth. This is because most assets on Booth—including the base avatars—require that both the seller and the buyer have bought all the assets.
However, you may have noticed that every once in a while custom Booth avatars have been ripped (from users who have made their own custom avatars) and repaired, then posted to this forum. Some prominent users who post them are Lotus Okami and Milk Drinker. I have also seen hints of a website that sells custom Booth avatars, but I haven't looked into it much after learning that it may require a chinese phone number to sign up.
If you wish to try your hand at assembling your own custom Booth avatar, tools like Modular Avatar and VRCFury simplify assembling an avatar greatly, and I'd be glad to provide more guidance.
