1988 Omni egr valve fitting

Welcome to Allpar. Are you talking about replacing the intake manifold or just the EGR plumbing? They may be metric or standard pipe thread.
Removing the manifold from the car may allow a machine shop to drill and re-tap the hole. Any K-car or 4-cyl minivan may have the same pieces.
You may still be able to replace the EGR tube.
Is it like this?: 31-139 EGR Tube.jpg
Or this?: getimage.php

No, not the big pipe. This is the rubber vacuum line going from the throttle body unit itself to the egr valve. The tiny fitting attached to the throttle body unit that the rubber vacuum line attaches to rusted out and broke off. Also a bit concerning why this would even rust in the first place. It looks like the fitting rusted almost completely shut and then broke. Car has idled rough since I bought it a few weeks ago, I'm sure this didn't help.

I can purchase a used throttle body/carb unit for about $75, but if I can simply replace the fitting (assuming I can get it out) that would be much cheaper.

The fittings are pressed into the throttle body. It may be a standard 1/8 " steel tubing with a bubble flare at one end to hold the hose on. Maybe drill out and tap in a scrap piece of brake tubing?
I can only think that the fitting 'rust off' was caused by condensation and exhaust byproduct acids being sucked in from a leaking EGR valve. Exhaust gases contain a lot of water. You might want to check and replace the EGR valve.

Thank you so much for the info that it is pressed in. I assumed that it was threaded and would have gone nuts trying to unscrew it. Given that the tbi unit was sucking in condensation from the egr and was also sucking in oil into the air cleaner box (from some other vacuum issue, perhaps?), maybe it is simply time to try a scrap yard replacement unit. The existing unit has had a rough life.