It's pretty easy to tell just from looking at the skewers which size you currently have:
15mm
9mm
I'll browse around some sites & see if there are any good deals on 15mm forks. It's been well over a year since I was on the market for parts so I don't know how much more common 15mm forks are now (if any), but when I got mine it was a really good deal @$320 (Marzocchi 44 Micro Ti).