I bought plenty of them at home Depot and Lowes for various projects, especially 1x3. They can be harder to locate, so you may need to ask for help. if you're needing treated wood, though, I'm not sure if they carry those sizes.

sometimes they call these boards furring strips... i think you can find 1x2s and 1x3s.

I'm at home Depot right now and treated 1x is on the same row as the materials for building deck railing, lattice, etc. treated 1x4x8 is $4.47, 1x2x8 is $1.97, 1x4x6 is $3.37. I can't find treated 1x3 so you might have to rip 1x6 ($5.98 for 8'). Halfway down the aisle pictured and most stores are set up similar.

thanks for the confirmation. My local HD is laid out a bit differently and had slim pickings for 1x. All I saw was 1x2, and every HD employee ran away from me like I had the plague whenever I approached for questions. They are so anti-customer now its ridiculous.

Curious did you see any untreated 1x that wasn't finish trim?

Anyway, I'll keep a lookout and stock up when I find some.

are the 1x's just for the siding? if you want to use untreated https://www.lowes.com/pd/Spruce-Pine-Fir-Furring-Strip-Common-1-in-x-3-in-x-8-ft-Actual-0-718-in-x-2-437-in-x-8-ft/1000016911

Any actual lumber yard will be able to provide you with 1x1, 1x2, 1x3, 1x4, 1x5, 1x6, 1x8, 1x10 and 1x12. The wood is frequently better quality than 2x4 material, with good (or better at least) faces on the boards and so will cost more. They will also have fence board material in 1x6 or 1x8 that will be cheaper and can be cut to any smaller size, with some losses for ill-placed loose knots.