EBAY, they have a few sellers selling new PRAM batteries there in any quantity you like. Renoldo something is a guy I purchased a bunch from over the years, probably $5+ each with shipping included and he gives you a $1 discount on the next order.
The square 4.5V batteries used in the 6400 and other models are $15 at radioshack, not sure what they sell the 1/2 AA 3.6V batteries for but they are most likely too much.
Batteries do cost a few bucks if you have quite a few machines (I think I have 20+ now), but it is just one of the costs of collecting. Not every machine needs a PRAM anyway, some need one just to boot (or 2 as with the mac IIs).
The square 4.5V batteries used in the 6400 and other models are $15 at radioshack, not sure what they sell the 1/2 AA 3.6V batteries for but they are most likely too much.
Batteries do cost a few bucks if you have quite a few machines (I think I have 20+ now), but it is just one of the costs of collecting. Not every machine needs a PRAM anyway, some need one just to boot (or 2 as with the mac IIs).