I like to use filler and flux by GasFlux, you can order then from
http://www.henryjames.com/
The type of steel, for practice, is not a big issue excepting it's dimensions and ability to be cut and mitered. 4130 is popular, any left overs can always be used for actual projects... Straight gauge tubing is available from a number of industrial sources. Bikes usually use .028" to .049" walled stuff. If you get a tube with .058" walls that's .125" larger in diameter then another tube it will slip over the smaller tube and have just about the right fit for lug like practice.
Are there any sculpture classes in town? How about race car building? The only guy i know of in your area is not a full time builder. Andy.