There's lots of options available in that size. Here's some examples of 37X12.50R15 tires for you if you're interested:
Maxxis Creepy Crawler
Interco Super Swamper IROK
Interco Super Swamper SSR
Interco Super Swamper TrXus MT
PitBull Rocker XOR LT
1010Tires.com lists 30 different tires in that size. And they're Canadian.