[mattbrewsbytes]

You'd probably be surprised how fast a hand saw can cut through a board.

Buy a handsaw at the big box store when you buy your wood, cut it in the parking lot before loading it into a cab/uber/rental car. Only do rough cuts (e.g. a little longer than you need) unless you have 100% certain dimensions. Buy sandpaper and sand your way to perfect fit at home, if needed.

The 4 36" dowel rods should be purchase-able in that size. You could ask the big box store to cut larger boards, they do this in the lumber area for plywood.

If this is too much to take on - hire a handyman to do the prop for the AC and ask for them to get the other supplies.

[Declanomous]

There's a saw at home depot (and probably Lowe's) and they'll cut your lumber for you. I think it's one cut per piece of lumber for free.

Home depot lumber is super expensive to begin with, so it's kind of baked in to the cost.