Fisher inearz would be a good bet, they have a couple of threads on here so do some research and should get the basics.
Basically you go to the audiologist of choice, have some impressions taken (following the instructions on fishers site)then send them and the iems into fisher (they are based in Florida if that matters) pay the $90 for the reshell and $30 for the cable (which is very similar to the westones so you should be ok), wait x amount of weeks and then get them back hoping they fit ok. If not you may need to send them in to be readjusted slightly (this happens, don't worry about it).
If you want "quick" you may have to pay extra for a rush build, up to you if it's worth it.
Have a look for those two threads there'll be a lot of useful info in there.
Edited by davidcotton - 1/21/13 at 11:33am