The Pure is a great keyboard and pretty much any ANSI key cap set for Cherry MX switches will work just fine.
The one hurdle you'll have is the "~" key and the "Right Shift" key. Those two keys are different from standard ANSI. The "~" key is typically a R4 key, but on the Pure it's an R2 and the "Right Shift" is too short (1.75 instead of 2.75). Below is a picture that better illustrates.
Not A Perfect Fit by
Michael Ashby, on Flickr
If you purchase key caps from a Group Buy, often times they will include a "Tsangan Kit", which should give you the extra keys you need. Take a look at the
RainDrop GB for example.
Unfortunately, I haven't seen any commercial key caps sets that include the Tsangan kits unless you buy a specific set from Vortex that is made for the Pure. Not sure that I've seen any either.
Hope that helps!