I bought my AK mags from the following places. I mention this because I see folks with new AKs always asking where to buy mags.
Each of those sites have reasonably priced magazines. They also offer a variety of types (polymer, steel, new, or surplus) from different countries.
Check those sites often, as much of their stock tends to be sold out due to the pandemic.
Keep in mind that cheap doesn’t always mean good. KCI mags tend to have reliability issues and are flimsy in build quality. Croatian mags tend to be hit or miss when it comes to latching in place (see here for my experience with the Croatian mags). The reason I wanted Croatian mags was for their bolt-hold-open (BHO) capability. Croatian mags are also not surplus…they are new.
Good mags that are usually cheap are Bulgarian surplus mags. I have not reviewed the Bulgarian mags I have but they just work and do not feel flimsy.
I also have 6 x Tapco polymer mags. Folks LOVE to hate on those. I didn’t buy them. They came with the first AK I bought (the AMD-63). They work, which is something the Croatian mags wouldn’t initially do. They will also fit in every AK I have, which is, again, something the Croatian mags wouldn’t do. They’re perfectly fine for range use and have yet to fail on me.
UPDATE (1/10/21): PSA has a sale on Croatian mags right now, marked down from $19.99 to $12.99. Also, if you order 10+, shipping is free (per the product page). As well, Mrgunsngear stated that they’ve changed the mag tab (probably narrowed it slightly) so that there’s less of an issue with latching.