they make disc adapters that thread onto the fixie side of a flip flop hub, but I don't know of anything for the front that doesn't involve relacing the rim. I remember back in the '90s there was a company that made adapters that bolted into the spoke holes, and then you relaced that side of the rim to the adapter. that's about the best you can do other than buying a new hub.
just go to your local bike shop with the rim and tell them you want a disc brake hub in it, they'll be happy to accommodate you for the price of the hub + about $30 depending on the shop.
More importantly, does your front fork have disc brake mounting tabs? if not, you'll need to replace the fork with one that does, and that will run at least $80