It is possible, but 95% of that there is unusable in games without editing. Even best “studios” that are doing commercial models often do not care about uvs, or merging objects or topology.
So even if you buy one you need to do editing, but if you can edit and fix such model when you bothering with buying it.
Best website for stuff that works in ue4 is marketplace.
There are also “Blendspaces” and you could try unity market.