Here are some options from an ebay vendor Backwoods Classics. I bought his kit for our '58 Thunderbird and have been very pleased. For Mustangs he sells a bracket for $100 that bolts to the stock drum spindle and uses modern caliper and rotors readily available from the local parts store.
The Scarebird swap and the Rod and Custom swap are the low cost leaders for this swap. (Just do a search on either name and you'll get a ton of hits.) Both use a bracket that bolts to your car's original drum brake spindles. Both use GM calipers, but R&C uses a Ford rotor, while Scarebird uses a GM rotor.
You may notice from my sig line that I have Scarebird front discs (he also sells a rear disc kit as well), but the design has been evolving since I put mine on, so it is not exactly the same as what you would get today.
Swapping the spindles "may" cause geometry problems in the front suspension, although most who do it have no problem at all. I think it depends on how wide of tires you end up running, and if you up grade the control arms or not.
In any event there are a number of low cost options out there now.
(There are also a number of high cost options as well if money is not tight - or is no object.)