I think you're right. The iMac uses the same mSATA blades as the retina MacBook Pros and the MacBook Airs, but I think you can't just use any off-the-shelf SSD, hence why OWC's kit is probably your best, if not only, bet.
If standard 128gb SSD isn't enough for you, or you don't even have FD currently, I'd suggest to buy a 256GB Samsung SSD pulled from rMBP on eBay. It's about 300USD and is the best price/performance option IMO.