Lets put it like this; if there are, you don't want one.
First off, I seriously doubt there's a transferable out there. In several years of playing the NFA game, I've never seen one offered for sale or heard on a transferable (and believe me, I've looked). If there is, there aren't more than one or two and the owners aren't going to easily part with them. For an idea of the price, consider a transferable Beretta 93R. On occasion, you'll see a transferable pop up for sale - so they are definitely more plentiful than G18's (assuming there are any transferable G18's) Last one I saw available was about $12k and that was quite some time ago (and it wasn't even a factory gun - as I recall, it was on a Taurus frame). At this point in time, I would expect a transferable 93R to go for $15k or more. Now, if a transferable G18 is even more rare, you're going to be talking about a hell of a lot more money that that. If (and I'm still making the dubious assumption there is a transferable G18 out there), but IF there's one or two in existance, they'll be significantly more expensive than a 93R - probably in the $20k range. For that kind of money you could buy two MP5's with all the fixins.
There's other options though. Transferable 1911's and S&W59's exist, and still pop up for sale. You could probably land a 1911 or S&W59 for just a shade under $10k.