I've heard after market are either hit or miss on reliability and the originals (if you can find them) are real pricey. I haven't seen so many holsters, but linked one below, might just want to search GB for "225 holster" and see what you get.
Mag:
GunBroker.com guns: Factory Sig 225 P6 9mm magazine NO RESERVE (item: 102978176 ends: Jun-29-08 02:53:58 PM)
Holster:
GunBroker.com guns: Inside The Pant Holster SIG 225 226 228 229 239 SI (item: 103622892 ends: Jul-01-08 12:47:43 PM)