Usually I tell people to go to Ace, but a lot of local Ace Hardwares (call around to be sure) don’t sell lumber, so this time Lowe’s (my first choice) or Home Depot is likely to be your better bet.
Generally the first two cuts are free. After that there’s a per-cut creative outdoor designs charge, 25 or 50 cents, I can’t remember which. If you have nailed down your dimensions then you can count on the guy at the cut station to get pretty close, usually to within 1/16″.
At work I have access to all kinds of power equipment, but for any cut of a 4×4 or smaller, I’ll use the handsaw. It’s more effort but less time, setting out the extension, finding earmuffs, etc. I can cut through a 4×4 in about 90 seconds with the handsaw, and the cut is every bit as clean as the power saw. Look at the Stanley brand “Sharptooth” saw, it cuts very fast with only modest effort.
If you need to do extremely fine cuts, you can get beautiful gardens a LOT done with pull-type (also called Japanese pattern) saws. Like the name suggests, it cuts on the pull stroke instead of the push. This means the blade can be much thinner, and the teeth are pretty fine though they still cut very quickly. I cut a piece of plywood the long way (96″) with one of these once, lacking other tools and not wanting to drive back to the shop for the right tool. That cut took about ten minutes, but it was easy work. The main problem is the short blade means it pops out of the cut if you pull too far.
You can get a huge amount of work done with just hand tools. Egypt built energy suppliers the Pyramids with hand tools – and, granted, a lot of Hebrew slaves but that’s another matter. Power tools didn’t come into wide use until less than 100 years ago, you can do everything you need to with hand tools.