What is the consensus on a replacement MAF for a 1991 EA82?  Subaru parts seem scarce - only found them on eBay.  Is aftermarket over the counter sufficient?  Auto zone, NAPA, etc?

I would trust used factory over anything from any parts store. MAF sensors are HIGHLY calibrated sensors and in virtually every case I've seen of aftermarket replacement sensors for older vehicles they are scaled incorrectly resulting in poor operation and huge fuel trims. OEM Subaru new or used only in this case.

GD

In general, for EA82s, you want to start collecting spare parts whenever you find them.  Many are NLA from Subaru or the aftermarket.

Using an aftermarket MAF sensor is a hit and miss.

This is specifically about EJ MAFs, but the same principles apply to any 'OEM vs. aftermarket' electronic component discussion.