Welcome to the forum Bob!
Try your local autoparts stores, if you have Autozone in Ohio they might have it (I saw it in one here last year).
Otherwise Amazon.com, Moss Motors, and Victoria British (among others) carry both the Haynes manual and the Bentley manual. The Bentley manuals are reprints of the original British Leyland/MG manuals with some editors notes at the end of the chapters to clarify some odds and ends and has a reprint of the owner's handbook in the front. They come in two editions, one for 1962-1974 (the green book) and the other is 1975-1980 (red book). I have both and have actually found the earlier manual to have some extra illustrations and information that translates to either, especially if you do any removing or modification of the emissions equipment.
The Haynes manual is a comprehensive, covering all years of the MGB and has some troubleshooting information for each section, but overall always seems a little lacking to me. I've found both books to be helpful, but if I had to have one or the other I think I'd order the Bentley book.
There's also a good restoration manual called the "MGB Restoration Manual" published also by Haynes. It has a step-by-step overview of a restoration done in the UK. It has a lot of information but isn't an in-depth "part A fits in slot B" type book... but it does show quite a bit of things taken apart and the process used to put it back together again.
Lastly, if you don't have one yet please order a free catalog from Moss Motors and/or Victoria British. Even if you scrounge for parts both these catalogs are terrific parts manuals with excellent illustrations so you can see whats what in the car over and above the workshop manuals.
Good luck --- Scott