Welcome @SentientPulse. Are you new to just Karting, or racing in general? Reason I ask is that starting out in a super is going to be challenging without significant racing experience under one’s belt.
To answer your questions: There really aren’t any websites for us based companies that I know of.
On Facebook you can join the American superkarts group and see what’s for sale. It’s pretty rare that one builds one from scratch by piecing parts together.
Engine wise, depends on your budget and the specific class/division you want to run. For the most part, there’s three main groups:
The single cylinder production engines (Called ICE) tend to be the cheapest, CR250 for example.
In the middle you have motors that are based on production models with superkart specific parts. An example here is the Viper cylinder and head for the CR250.
On the high end of the scale you have twin cylinder motors from moto GP based ones like the TZ250 and RS250 and kart specifc ines like the GasGas.