There will no doubt be places in the US that offer both.

But Capricorn, Arrow, Farndon might be some UK sources

The biggest question is, do you really need them ? What sort of rpm will you be using ?

Steel rods may be heavy, but you know 100% that it is super strong. I'd be looking to lighter pistons first.

Titanium is strong, not as much so as steel, but of course it is much lighter. Cant say Ive heard of it being used that often though even in high rpm screamers

Alu/GRP as they call them. These are light, but they are also huge. The proportions are always much larger than a similar steel rod because the material is weaker and needs to be larger to be strong enough.
They are also alleged to deform over a period of time. Although that might be because the people who use them, use them in extreme duty scenarios. ie top level drag racing so they are making huge power for short periods. So they almost replace them as a service item. Just check for used ones for sale, they are readily available and quite cheap because of this. Ive a big alloy conrod sitting beside me at the minute from some sort of drag car.
Same as these

http://www.ebay.com/itm/BROOKS-88-AL...-/250898078697

Or smaller SBC ones

http://www.ebay.com/itm/Manley-Small...#ht_500wt_1076

They are so cheap used, because they are deemed beyond their service life.