There's a French company, Hub Le Bas, who produce an alloy steel tube from their own material designated 15 CDV6. It's similar to the English T45 and the US 4130, but doesn't require any heat treatment post welding, even with MIG.
It would be ideal for DIY wishbones etc. as you'd have very tough yet stable components without all the hassles of welding some of the other exotic steels.
Problem is, I'm not sure if it's actually sold outside France.
There are Hub Le Bas depots around Enfland (Bilston, London, leicester, Manchester and Paisley), so it may be worth enquiring.
They also stock huge ranges of circular, rectangular and square ERW and CDS in both metric and imperial sizes. Hub Le Bas.

I use Hub a bit, and we supply them from where I work (Timken Alloy Steel Europe Ltd, formerly TI Tubes Ltd, Desford).

Hub don't make them, they are a stockist, but yes, very good availablility of non-std stuff.

[Edited on 11/4/05 by NS Dev]

Hey NS Dev I did some Control System/SCADA work at the Timken Faircrest Steel heat treating Plant in Canton, Ohio few years ago. Timken is a very well known local company. I was amazing to see some of the old equipment that they were using for heat treatment then. The operator would walk past the temperature display every hour and decide if it had been at temperature. When the SCADA system was done you could tell if cold air was entering the furnace by the amount the gas valves were open in different places in the furnace.