I've done it a few times on both my Gen 2 and 3 N/A's now.
I took most of the intake system off to do it (just because the space is good), although I left the manifold itself on the car.
It's a PITA to remove the manifold (ideally engine out is the best way to take it all apart), and you can do it with just the plenum off if you can fit a socket in there.
I got a genuine rocker cover gasket set from NZ for ~$50, but that was a one-off. Going through CMI Toyota will have you paying at least twice that, and you'll be waiting for ex-Japan stock. So minimum 3 week wait for it. You CAN get them through Supercheap Auto/Sprints/REPCO(maybe?) as well. Although, since it's not genuine, there's no guarantee for quality there.
they ARE a lot cheaper, though. And will usually arrive in a couple of days.
AFAIK, then Gen 2 and 3 gaskets are the same, which means you'll also be able to get Celica ones, and I think even the 5S gasket might fit (Don't quote me on that). There's a lot of interchangeable parts between them.
This is my experience with the gaskets. Maybe in different states you'l have more luck with getting a genuine, but in SA, pickings are slim at best.
Hope this helps!