Have any friends? Any local facebook car groups? Maybe a gas station compressor? A craigslist ad asking for the use of a compressor for 15 minutes in exchange for a beer?

What everyone else has said (get your own diagnostic gear and an air compressor, seriously) but I wanted to add a few thoughts:

If the miata is your only car, why are you wrenching on it?

I fixed worse engine problems than this working on my back in a dirt driveway in a tiny rental property. I had an air compressor and a full set of tools in my living room. My landlord was not pleased when he came over and saw all the parts of the engine in little labeled plastic sandwich bags on the floor. But I still got the car fixed and it ran afterwards.

If you have the engine apart at the moment:

• Check the block and head for squareness with a straight edge- along the 4 edges, across like an X and across the middle both ways. Measure with a feeler gauge but honestly it should be obvious if it's not flat.
• I would put the hose fittings on the coolant passages on the head, cap them and put some compressed air into the coolant passages of the head... not a ton, but enough that when you spray soapy water onto the cylinder side, it makes bubbles where the crack is.
• If everything is flat with no cracks, you should carefully reevaluate your head gasket install procedure. Biggies include- use the MLS one, no RTV, correct orientation, torque down the bolts/studs spiraling from the center out in 3 stages of tightness. Use a beam torque wrench, don't leave out washers and stuff under the head bolts, properly lube the threads with ARP lube, etc.

If you have it currently together, take off the cap to the radiator, fill up the coolant and do a leakdown test.

• The cylinder(s) where the leakdown test sends bubbles out the radiator cap is bad.
• Take the head off, check everything for leaks and squareness as above.
• If the block is fine and the head is a bit messed up, take head to reputable cylinder head shop, have them flatten head, fix the valve seats, etc.
• Reinstall with fresh MLS head gasket and ARP head studs, torque to 65 ft lbs (or whatever they say, if it's been updated).

You're absolutely right. Not looking forward to what my wife has to say about another piece of equipment inside though. Gotta be done I guess.