Brian, here are a couple of suggestions.

First you can get unfinished discs from workshopsupply.com in Canada. That might
be a good starting point.

As pointed out, you have a wide selection of topcoats...urethanes, lacquers,
shellac. All have different applications and different application methods and
different solvents. Cutting the finish with its solvent to get a superthin coat
isn't necessarily the best thing to do. The manufacturers will recommend levels
of dilution and you really should stay close to those. Finishes such as shellac
and lacquer dissolve the previous coat to bond with it. So too much solvent can
really cause deep problems in the finish. Finishes such as the polys require
sanding between coats and the new coat bonds to those scratches...it doesn't
dissolve the previous coat. But coats of poly that are too thick can cause
problems with the underlying coats, too.

The final deed is the polishing. This really makes a difference. When we polish
our boards, we start with 220 grit, then go to 360, then 500, then 1000, then
2000 and then 4000 and then polishing compounds and then a high carnauba wax.
Grits in these levels are hard to find at your local hardware store. You have to
go to someplace like woodworker.com.

Here's my finishing suggestion. Do the final topcoat and let the discs cure for
as long as you can. At least 24 hours, but the longer the better. Find a piece
of 1/4-inch plywood (or anything slightly thinner than your discs) and drill 24
holes in it 1 3/8 inches in diameter. Screw this polishing jig to a workbench
and drop a disc into each hole. Now get out your random orbit sander with the
superfine grits and start polishing. The jig will hold the discs from flying all
over the room (under the freezer and places you can't reach). Of course you'll
have to do the edges by hand, but those are not as critical to get supersmooth.
If you don't have a power sander, you can wrap the sandpaper around a block and
do it by hand.

The thing about final finishing (or rubbing out, as it's called) is that this is
when you really finally see the results of your labor. It's also the time when
you see the flaws. Something you hardly noticed at the beginning stands out
horribly when you're nearly final rubout completion.

Oh, and one last thing (I promise). Some discs are convex on one side and
flat/concave on the other so there is a "fast" side and a "slow" side. If you
were to use prefinished discs and sand them back to raw wood, you could be
affecting that.