Local classifieds, both print and online, are popular options for finding unfinished kit cars. Thoroughly inspecting the car is vital, as you are likely on your own when it comes to finishing the car, especially if it's an obscure kit.
Ensure the parts needed to complete the kit are either included in the sale or easily procurable. If you need instructions, make sure you can get those, too.
The effort, or lack thereof, that the seller put into the kit before selling it can directly affect how smoothly your project goes. When looking at a car, pay attention to its surroundings. Is it sitting in the middle of a garage, or in the middle of a field? Are the unassembled parts neatly organized or nowhere to be found?
Many car forums have some sort of vetting system, so you can visit one dedicated to kit car dealers and find an honest seller. It is also relatively easy to get extra pictures of the car.
Online auction sites have large selections of kits, but they require extra caution. Some sellers do not offer refunds, so if there's no way you can see the car in person, get as many pictures as possible. A reputable seller knows what buyers are looking for and should gladly oblige you.