There was a DVC discount for Alamo and National but I do not know if that is around any more. The last time I recall even checking into it was about 10 years ago and at the time you could get better deals without using it. There is and has long been an Alamo discount if you have an Annual Pass, which usually suffers from that same problem of being worse than other deals you can find.
There is a site dedicated to things Disney, which I am not sure I can mention on this site, which has discount codes you can use for the various rental companies.
We have usually found the best rates for cars that you can pick up at the airport with Dollar, not even using a code, but you need to be diligent, e.g., Dollar often shows rates that we consider too high but then for a couple of very short periods, often lasting for no more than a day or two online, between six and four months out, the prices plummet for the time we want, e.g., often by half or more for a week rental, and then go right back to the original prices after that short period.