I use a 15/16th one out of some Corona. Just had to redrill the mounting holes to bolt it to the booster. Heard the Starion ones are good too. But go brand new. Not worth fitting a 20yr old unit thats got rust in the bores already.
If you take your stock AE86 master down to a good brake joint they should be able to measure up the crucial dimensions and recommend a bigger master, or check that the one you've chosen will work.