Yep, these adapters are just like the regular 12mm hex hubs, but they are 14mm instead to fit inside the Maxx wheels. There are several places to but them, but for your setup, you would only be able to put them on the rear since the front bearings differ from those used on the Nitro Stampede. Here they are:
http://www.ultimaterc.com/forums/showthread.php?t=31229
http://maximizerproducts.com/mtr.html
SH Z-Car, Custom Crawler, 8s Savage,
12s XTM XLB 1/7 buggy,
4wd 4-link rear/IFS Pro4 truck,
Custom Hyper 10 Short Course,
Belt-Drive Mammoth ST 1/8 truggy, 4s 17.5 MM Pro HPI Blitz