It's pretty nigh impossible to get Heavy Gauge strings nowadays, Mediums (usually 13-56) are as heavy as it gets.

You don't say where you are? If in the UK, Eagle Music sell their own sets of Newtone DADGAD sets which are a mixture of light and medium gauges. THey're in Eagle-Newtone packaging, but they're Newtone strings (which means very high, hand-made, quality), and only around 6 quid a set.

Otherwise, if you're not so finnicky, a set of standard mediums (13-56) will work fine. I use lights on my OM for DADGAD and they're **OK**, but mediums feel better.

Usual disclaimers apply - IMHO, YMMV, IOSBTBDO, etc.