A possible solution in a pinch is to get an already flat surface, ideally larger than your board. Cover it in something that will transfer (ink, paint, toner etc). Rub the face of the board you want to flatten across your flat surface**. The** transfer substance will pass onto the high spot in your board. Scrape, chissle or sand the high spots down slightly.

Repeat this until most of your board is marked by the transfer substance. Your board will be mostly flat (or at least as flat as the reference surface).

This technique is used in metal work, but it's labour intensive. For woodwork to achieve sufficient flatness planes are quicker and produce a better surface finish. But if you don't have any large ones, this method might work if your desperate and don't want to buy new tools.

For a less accurate flatness. Place the board on the flat surface and push in the board to fell the points in contact with the flat surface. Then take those parts down.

The solution I have found is a sander and realizing you will never be perfect.

Look for the imperfections in the garbage they sell at the store. The bottom of your kitchen table. The inside of your kitchen cabinets. Those are the mistakes they're trying to hide.

There's a pretty easy solution if you have a decent plunging router with a good flattening bit head.

I set up two 2x6" along the length of my board I want to flatten, and then made a jig box for my router. The jig box is able to slide back and forth while resting across the 2x6, using the depth guide to keep the cuts at a level depth as you do your pass overs.

There are some professional woodshops or wood suppliers that will run wood through a planer or drum sander for a fee. I have seen $50-100 for a table top size slab, double that if it has resin. So call around.