What type are you looking for, and what's your budget?

If you want the cheapest damn gun possible, pawn shops tend to have single shot break-action shotguns for $50-$100, and likewise have single-shot .22 rifles. Not great for defense or revolution, but better than nothing, and affordable to almost anyone.

If you're looking for an all-around doer at cheapest price, get any number of used pump action 12 gauge shotguns, available typically from $150 to $200. With a shotgun you can use various types of ammunition for various purposes, including small game hunting if that were a factor, up through and including defense against humans or even large animals/unarmored vehicles.

If you go up a little from there, you can get into some decentish rifles for defensive use, namely hi-point carbines and SKS rifles, the former being 10-round magazine-fed semi-automatic rifles that use pistol cartridges, and can be had for $200-$300 in most places, and the latter being soviet and chinese surplus military semi-automatic rifles fed from 10 round stripper clips and using the same cartridge as the AK-47, so a bit more powerful, but unfortunately costing a bit more at $300 for a bottom tier and battered chinese example up to $500 for a nicer russian or even more for a yugoslavian.

For pistols, you can get bottom tier defensive guns pretty cheap, with the heritage rough rider .22 revolvers going for $100-$200 but in a poor cartridge for defensive use, and being limited in single-action functionality, but more ideally there are the hi-point handguns in more standard defensive calibers for $150-$200, and Rock Island Armory double-action revolvers for $200-$250. If you can do $250, you can also get the sleeker and more concealable Bersa Thunder in .380, which while not ideal is an acceptable defensive caliber.

For handguns that are more war-bound and have higher capacities and larger size, Taurus P92s, surplus Glocks, and various Turkish knockoffs of brand name pistols can be had for $400-$600, but this is less important generally than a good rifle.

For a truly worthwhile rifle, I suggest getting something fairly standard so you can have good availability of parts and ammo: that means AR-15 or Kalashnikov designs, both of which are semi-automatic and typically hold 30 rounds in their magazines (though there may be legal limits on magazine size as well as on these rifles depending on your state, so be sure to check your laws) and in decent intermediate rifle calibers. This is what the militaries of the world give their infantry (more or less, theirs have the option to go full-auto, but ask any veteran and they'll tell you that they almost always used them in semi-auto) as well as what most of the right wing and fascist militias carry. This is what you want, if you can afford it. Low end models go for $500-$700

Also worth getting is body armor. Decent level III steel plates (which will stop most rifle rounds, which soft armor will not) can be had on ebay or through other internet sites for $50-$80 per plate, and dropped into a carrying vest that costs roughly $50 to provide front and back torso protection from most bullets you'd ever encounter. I'd recommend getting armor even if you don't get a gun, as it could save your life at a protest if it suddenly turns violent. Be aware however that it is restricted in some states, and committing certain crimes while wearing it is an additional felony, so be careful out there.

Regardless, remember to get proper training so you are both more effective with your gun and not a danger to yourself. Don't be a hero, don't be a martyr, a gun doesn't make you a god and a living comrade is more useful than a dead one, so use your gun only when prudent and necessary, and try not to break any laws because prison isn't worth it in almost any situation you're likely to encounter, and likewise be sure to study the laws in your state as they vary wildly and it's very easy to accidentally become a felon if you're not careful. A gun is just a tool, and one that will help you to stay safe in unstable times, but only if you recognize it's limitations and your own, and remember to use your brain. That said, welcome to the club, the left needs more armed proletarians.