A cheaper option would be using a benchtop thickness planer (about $600 for a good one) and use a jack plane to flatten the other side. Ie. flatten one side by hand (doesn't have to be perfect finish, just mostly flat with no twist or cup), run it through the planer, then flip it around and plane to final thickness. It's a pretty sweet spot when it comes to tool costs, space requirements and time investment.
Jointing and planing is something I have an issue with being a primarily hand tool woodworker. I don't have a proper shop space so I can't be buying stationary equipment. I do visit some shops occasionally, and for my next project I intend to go to a shop with good machines and do all my jointing, planing and dimensioning. It's not very convenient (far away) or cheap (hourly billing), but it'll work.
For my previous project, I hand planed some of the work pieces all the way from rough lumber and it wasnt't that bad. But it did take a bit of time and my end result wasn't perfectly flat, square and parallel. But it worked out and the pieces were too small to go through a jointer and a planer anyway.
Buying pre-dimensioned lumber is also an option but a very expensive one. And wood doesn't like to stay flat when humidity and conditions change, so a minor touch up might be required.