Tape substitutes to prevent flats in mtb inner tubes?

They're puncture resistant-ish, they can be a faff to install, they will increase your rolling resistance... I had them in my commuter's tyres as the spikes came through the tyre and pierced my tubes, but it's more a stopgap.

It WILL protect against something coming through your tyre, however with MTBs the most common punctures are snakebites, and they don't protect against that. And when you get a snakebite you'll have the additional faff of putting them in again after you patch up the tube. They do kinda work, but the question is, is it worth it? To me, no.

They're more often known as tyre liners, and are usually polyurethane, about 2mm thick.

I've used them on a cheap bike, that was worth less than a decent set of tyres, and eventually had trouble with the end wearing a thing patch in the tube - but they kept the broken glass and other debris out successfully. Tyre liners are already the cheap option.tyre liner material

I'm not aware of a substitute source of the same material, which is rather thick, unlike most things sold as "tape". What people have been known to do in the past is put an old (unpatchable) inner tube, with the valve cut off, into the tyre before installing the real tube. That effectively triples the thickness of the tube where it matters. PU sheet that I can find easily is far more expensive in practice, because it's sold in large pieces

It should be possible to put an old tyre in there, with the beads or the whole sidewalls cut off. That would be thicker still than the old tube, and of course is tougher rubber. You'd need to cut across it so it would fit flat, and probably trim off the overlap. This could be a bit of a pain to fit.

Generally speaking the best option is puncture-protected tyres.