[uponthestair]

Thanks a lot, so the 18 round will fit without any mods right? You meant the pads are for modifying my own current magazines to hold 18?

Yes.

Here are your options.

Canyon Creek Mags: XD .45 acp 140mm magazine - Canyon Creek

Canyon Creek Base Pad kits: 45 Acp base kit - Canyon Creek

Arredondo Base Pads (springs sold separately): http://www.arredondoaccessories.com/category.cfm?cid=1001,2015&PID=fi22l2540wnvhs&GID=XDBP

AGP Base Pad kits (includes spring): AGP Arms Springfield XD45 / XDM45 +3 Magazine Extension - AGP Arms, Inc. Store

My vote goes for AGP for $20. Try buying one and put it on one of your mags to see how it works. Then buy more if you want.