Hey my cassette is worn out and I'm looking to replace it. The stock part on my 2014 REI Novara Buzz is an SRAM PG-830 11-32.
Only ones I'm seeing are PG-820 11-32 and PG-850 11-32. Is PG just the part number or are those incompatible with my bike?
Also are there any Shimano casettes (for example) that I could use as a substitute if I can't find the SRAM part I need?

PG 850 is lighter, but any of the Srams will be compatible. According to Sheldon Brown, a Shimano 8-speed cassette's spacing is the same as Sram.

Ok thanks

If cassette is worn out and chain has not already been replaced recently, chain most likely needs to be replaced too.

Buy any Shimano, Sram, Microshift, Sunrace etc. 8 speed cassette with the spockets you want.

SRAM uses the PG as an acronym for "Power Glide", their marketing term to imply a smooth transfer of power.
In your case (PG 830), the first digit 8 implies 8-speeds and the last two digits 30 imply the grade of cassette. The 830 is low(er) cost, the 850 mid-cost and the 870 is high(er) cost. But all are 8 speed cassettes that will be a direct substitute for you.

The 11-32 is the teeth range of the cogs, smallest having 11 teeth and largest 32 teeth. If you keep this range the same as stock, then you won't have to shorten or lengthen your chain for the new cassette.

(The SRAM 10-speed and above cassettes uses a four digit code, like PG 1070 (10 speed, better grade than the PG 1050).)

That said, any 8 speed cassette made by SRAM or Shimano would work fine. The higher grade (high cost) cassettes typically are lighter and may have a different material and finish. However, all 8 speed cassettes by SRAM or Shimano have same fit/form/function. There are other brands out there, but I only have experience with SRAM and Shimano.

If SRAM is like Campy or Shimano, the higher grade cassettes get expensive really fast. I usually go for the lower grade Campy, either Centaur or Veloce, since I'm not racing. On the usual ride I'd never notice as long as the shifting is smooth, which it is. You would never notice on your bike either so get the less expensive one. And yes, replace the chain too. KMC is fine.

Yes, SRAM follows that business model, too. The high end SRAM PG1190 cassette is machined from a single block of steel and cost about $350. They start with a piece of steel, mill it down to what we recognize as a cassette, and there is likely more wasted material than actual material in the final product.