Here's one from Blade-Tech that's supposed to work just fine with or without a belt, but looks a little roomy if you're just trying to use it under the belt.
http://www.blade-tech.com/Single-Mag...product_images
Of course, if your desire is to wear it IWB, specify your weak hand when ordering, so the mag can go bullets forward when you reverse it to put it IWB.
Then, of course, there's always this little guy from Don Hume:
http://www.donhume.com/Products/Prod...?ProductID=122
I've been wearing a 715M holster for my P239 I received just the other day, and I can tell you - their clips are STRONG. Enough to keep the weapon from wobbling too much, so a magazine will be no problem. Might be a little bulky feeling, though.