Here's their website with contact info and a downloadable (pdf) 2011 catalog:
H&K 22 Rimfire - Home
HK MP5 10-Rd Magazine: 2245310
HK MP5 25-Rd Magazine: 2245311
HK MP5 Fixed Stock: 2245332
Heck...just Google '2245311 magazine'
Okay, Google suggested sites seem to be out of these mags. Arrrrgh...