Just buy nice handkerchiefs whenever you see them. I don't know about you but spending $25 or even $12 on something that just peeks over your pocket is a little silly.
I love pocket squares. I think they complete the look of the suit in the front. I'm always looking for stuff I can use as a pocket square.
You should always have a bunch of plain, white squares. They go with everything. I bought a box of them from The Bay. I prefer cotton since I like plain squares or simple peak shapes. If you're a more "poofy" pocket square dude, buy silk.
My go-to place for pocket squares? DAISO. Not even joking. I've had people ask where I get them from and they're always surprised when I say Daiso. They sell a ton of handkerchiefs for $2 each in all sorts of patterns. Some of them look horrifically ugly but remember that only a sliver of it shows. I've got some plaid ones from Daiso that look great on plain grey or navy suits.
I've even used a white napkin as a pocket square before. Unless you're Italian and can pull off the poofy pocket square look, keep it simple.