Lowes Hardware carries the 528MPK valve, which is like the 528T except it will work with more toilets.
If you have a Drake, it's the Red Flapper, also sold at Lowes.
Thank you! It's actually cheaper at Lowes-- I hope that they actually have it in stock up there. Those site to store things where they say it will be available "today" never actually work out that way in my experience. I wonder if I will even need a new flapper if the old one is in good condition. I suppose I can try it out and if the old one doesn't work then I can get a new one. I should probably pick up a new one and keep it on hand just in case.