This thread might really be helpful!

Arteria3D
I have the lifetime membership which I bought long time ago for a "few" bucks. That made me really happy. What I got is access to a HTTP-directory which is sadly messed up like hell. I asked the owner Steve for an ordered list and he tells me several times that it would be started the next week. Nothing happend so far, one year has passed from the first time I asked
Furthermore, I have a problem with publishing the models in MDL-format which is not allowed as Steve told me in a mail. Now, he luckily changed the license on his page and everything is fine. Steve personally is pretty friendly and you can talk to him about your problems and wishes. AND the models have a good quality and are really cheap. Unfortunately, models do not always come in mdl format.

3dFoin
I like this small provider a lot since the quality is "good", models are cheap and they come in mdl format.

Dexsoft
Dexsoft has the best support I ever experienced. I had a difficult licensing situation and Frank wrote me a special license based on a structure we discussed by mail. The models are not the cheapest but the quality is good.

The Game Creators Company
Models are cheap but the quality is not the best. The support sents prepared and no personalized messages.