What kind of batteries do you want? A123 are probably the best compromise of ease of use (not as finicky and, indeed, prone to blowing up if misused as lipo, and more energy density than nicad/nimh) and you can find them most commonly on ebay - you'll need to solder them up into packs though, have a look at http://www.terrorhurtz.com/a123/build.aspx for a vague overview of how to build up one style of battery pack. You'll need a suitable charger too.

What kind of metal are we talking about? Hardox? Metallica albums? 6-4 titanium? Mild steel? Bog standard (1000 series or whatever) aluminium? Engineering aluminium like 6061-t6 or even 7075? Tinfoil? You'll be getting different materials from different people probably - and then there's always the scrapyard