What are the best ulcer supplements on the market? Preventive or to help cure? Thanks.

I use Suceed. Vet recommended it. 

I use MVP Gastro-plex for prevention & compounded omeprazole for treatment. Add sucralfate to the omeprazole if I think they have hindgut ulcers. If you treat, make sure you start the preventative when you start treating, otherwise the rebound can spawn new ulcers within hours from the acid overproduction. A lot of people swear by the Succeed, however its not marketed for ulcers and the company tells you to get with your vet for proper ulcer treatments. But people swear it works, I've only ever used it after two major 2 week long impactions where my horse wasn't allowed to have any food for 10-14 days, I give it during the refeeding process because it takes about 4 weeks to get on a normal diet again. Actual treatment Rx drugs are cheaper so I stick with those for treating & then they go on Gastro-plex, it has hindgut protection as well 

Compounded Omeprazole. Your vet can order it from NexGen or any other pharmacy.