How much do you need?
If a reasonable qty a few full lengths, will be better off ringing a few steel stock holders, yellow pages! the price will vary tremendously as most of them either have a minimum charge or just cant be bothered unless sell by the tonne, their loss.
Swift steel are based in Cardiff and Dyfed steel deliver in Newport everyday. Full lengths only I think or try local engineering firms, I know there is one by the transporter bridge.
I 'acquired' some of this from the Highways Agency. They left a large road sign A frame in the hedge by the side of the road. After a year of it being there I assumed they no longer wanted it and their loss was my gain. It was a bit rusty but cleaned up OK and I had enough material to build my welding cart.
Not suggesting you start dismantling your local road works though !!
from a place called cwmbran steels... there's loads of other places in newport if you do a google search for steel stockholders newport. i went for cwmbran steels as the price was good and they had the material in stock ready for me to use on the weekend.
one place i phoned wanted £25 per 7.5m length!!
yeah ill update the list hitch, i think i'll have to tstart keeping some sort of cover in the car, it seems to be getting stinking too often!