I have to second Don Teach's recommendation of KS Metals.  This is
the same company that supplies hobby and hardware stores with brass
aluminum and plastic tubing angles, etc.  They have very reasonable
pricing and excellent quality.  Even with a typical retail store's
markup, the tubes (etc.) are quite reasonable if you just need a few
pieces.  You can buy directly from them but they probably have minimum
orders.

They also provide cutting and forming services and their prices,
quality and reliability are excellent.  It is rare to find a company
that provides the quality of service that KS does.  Unfortunately they
cannot make tee fittings.

Best regards,
Spencer Chase
Garberville, Calif.
http://www.spencerserolls.com/