Just use standard factory (made by Mec-Gar) P11 magazines, which fit and function perfectly well in the P15. They're 10-round mags and look and fit exactly like the factory 12-round magazine for the P15 ... which is, in fact, also a factory P11 magazine. (In fact, if you look at the side of a P15's 12-round magazine, it's stamped "KT-P11-12" for "Kel-Tec P11, 12 rounds".)
I would STRONGLY recommend only using the factory Mec-Gar brand mags and not the crappy knockoffs made by ProMag, which are notorious for functional problems. Also, while the mags made for the SCCY CPX-1 and CPX-2 and DVG-1 pistols might LOOK the same (because those pistols are knockoffs of the P11), they DO NOT fit in the P11 - the magazine catch hole is in a different location.
I would STRONGLY recommend only using the factory Mec-Gar brand mags and not the crappy knockoffs made by ProMag, which are notorious for functional problems. Also, while the mags made for the SCCY CPX-1 and CPX-2 and DVG-1 pistols might LOOK the same (because those pistols are knockoffs of the P11), they DO NOT fit in the P11 - the magazine catch hole is in a different location.