I find aluminum welding is always a big pain.

6061 tubing should be generally weld tolerant. Some fillers are designed for heat treating, others are designed to use as-is without heat treating, but you will lose some strength if you don't heat treat.

7005 is a bit stronger, and will allow for a lighter frame, but 6061 is still a commonly used metal.

You can purchase ready to weld aluminum tubes and small parts.

TUBES ALUMINUM :: Nova Cycles Supply Inc.
ALUMINUM TUBESETS :: TUBE SETS :: Nova Cycles Supply Inc.

Probably quite a few other suppliers too.

If you're building a standard triangle frame aluminum frame, then it might be good to start with a tube set. Go with your own if you're doing something very custom like a cargo bike.

However, some cargo bike builders will also look for good donor bikes for things like forks and rear triangles.