bananahamuck: I did buy a roll of cork gasket material from NAPA. ($9.02) Yes, making my own will be easy. In this case, the transmission was very clean inside and out - and was evidently "harvested" from a low-mileage 510 quite some time ago. A replacement cork gasket will certainly last for as long as I will need, and just feels better to me than using (even modern) goop.
As for the cost of buying from the online seller, it is really a matter of principle for me. I have purchased a lot of parts (for my Datsun Roadsters) from this source, and it just seems like the recent price increases are somewhat excessive. My hope was one of the typical aftermarket gasket providers would have a listing for such a (formerly) common vehicle, but as you note, the newest Dimes are 50+ years old now. Related to this, it is also a common experience for me to find an online seller has simply found a supplier with stock, buys them on the cheap and quadruples the price. Such is life. One would hope the Datsun community would look out for each other - which it does, for the most part - but self-interest is a motivator for many.