the Nissan dealership (don't laugh, they might actually have it)
or
Ebay (they pop up all the time. Search 'datsun 1200')
or
Nissan Motorsports

The dealer would be a good place to start, either them or a real auto parts store.
and IF they somehow can't figure out what a 1200 is, the ones used on the early B210 will work (up to mid 76 if I remember correctly). Wagner, Raybestos, EIS, Beck-Arnley, and several others still show them in thier catalogs. whatever you do, unless absoulutly necessary, get new ones, they won't cost you that much more and you'll know that you can trust them.
aluminum brake parts have a tendency to corrode and pit over time, this is because brake fluid absorbs water and the water causes corrossion

I'll second that. Get new cylinders rather than rebuilding the old ones. No worries about leaks. (this is even more important with the clutch slave cylinder).

I'm almost positive all B210's and 210's have larger rear wheel cylinders. Externally they are identical, but the bore on the B210/210 wheel cylinder is larger.
These will cause your rear wheels to lock up first under heavy braking ... say, if a few deer jump out in front of you... on the freeway... and posibily cause you to slide into a guard rail.
Been there, done that.

Nissan motorsports
look in BRAKES under Alternate wheel cylinders. I believe 11/16's are stock for 1200's.
Also when looking for 1200 parts, remember that some parts are listed under B110.

Here in Oz they are JB2311 and I sell them for $13 each

According to my U.S catalogues
Raybestos 37407
Bendix 33731
Autospecialty WC37274


I don't have a Wagner book sorry.
These are the 13/16 wheel cylinders.

...or on eBay try "B110" in your search.

tah dah!
rear wheel cyl on ebay

here's some more
more rear wheel cylinders on ebay