They are from a pre 2005 aprilia RS-125. You won't find any for sale here in the US (you can sometimes find complete bikes, but you won't find any being parted). I got mine from the UK, but you can check any European eBay and they are plentiful. I think I paid around $250 shipped. They are a nice swap since you don't need to mess with the stem, just requires new upper/lower stem bearings, and left and right wheel bearings. You will also need to cut new bearing spacers, one for the inside then 2 for the outer.
You can get any set of forks to work, but these are probably the easiest and cheapest, while still offering decent performance.