The official AS complete tank to bowl set including the big gasket for the flush valve: 738756-0070A, but it sucks that you would have to pay $14 when only one washer is bad. However, the repair parts diagram only shows this kit; nothing seems to come separately.
That said, I seem to recall a plumber on here saying that he tosses the AS tank to bowl set because it sucks and installs the toilet with a good Wolverine Brass tank-to-bowl set. You can't buy that, but it suggests that a good-quality brass tank-to-bowl set would work. If you get the official parts online, that's certainly easy, but I would consult a plumbing supply place about a good solid tank-to-bowl set that would fit this toilet (preferably a double-nut setup where you attach the bolt to the tank with one nut and the assembly to the base with another. [ E.g. in tank goes bolt head and rubber washer, on other side of tank goes metal washer and nut, then insert same into hole in base, and attach rubber washer, metal washer and second nut.]_Of course, if I recall, the crappy design of this toilet makes the nut very hard to access, so you may need to be sure that you have a long socket that will fit the nut.
Sorry can't be more help. I'm sure one of the pros will have more ideas.