I can't say bad things about letting others do the first build if one has limited experience.
I am in California and the only "retailer" I know of is Motorized Bikes | Motorized Bicycle | Gas Bike | Gas Powered Bike | Bicycle engines
I just finished installing a second motor on the Giant Boulder I own today so I now have more experience with "building" a bike.
It's simple and complex all at the same time. Simple if we know how to or have help in doing.
This site is excellent. Good people.