Hi Mike,
Had you thought to look in your local "yellow pages" under "steel stockholders". Don't be put off by the steel part, check individual adverts to see if they stock the other metals you want.
In West London , Acton, we have a really helpful place for the more common metals, naturally they cut to size and can fold sheet too, while you wait .
There is also the Metal Supermarket chain, but the prices can be the wrong side of surprising! They do though often stock the more esoteric grades of steel, but in a limited range of sizes.
The usual M.E. suppliers do a good job, but buying locally can sometimes save more than postage, if you know what I mean.
especially if you buy trade, in 3 metre lengths, which can be worth it if you are going to use a lot of one size.
chriStephens