You could find a shop that has a thickness sander and ask to have them run it for you.
Or buy a few hand planes and cabinet scrapers and make your own shavings. A #4 bailey plane with a slightly convex blade to rough it out and get it true/flat (planar in geometry) and then a #5 plane to finish it very smooth and get rid of the rough-planing marks. Finally a cabinet scraper for the final smoothing. You won't believe the upper body you can develop planing hardwood for a few weeks. You'll also discover if you are prone to tennis elbow.
Casey

I will first dovetail on what casey said. Hand planes are something that last a lifetime and can be very satisfying to use if they are good quility and you learn to tune and sharpen them. It go's alot smoother if you have a mentor from the begaining to help select good planes.
I wouldn't cut a slab just to fit it through a planer. As casey suggested,have someone run it through thier large planer or drum sander. Depending on a few variables slabs can warp or split. That only adds caracter and is easy to remendy if nessary. My favorite method is useing a router because it will do slabs that power planers can't. An example is where the log is split leaving you with a half round that is impossible to power plane but is a piece of cake for a router.. Go to magizine's online like "Wood" and "Fine Woodworking" and search instructions for building nessary sled for router.
I find it easier to debark logs with a pressure washer and "TURBO NOZZLE"before taking it to sawmill. If I get a slab with bark intact I usually proceed with project unless bark is already starting to slip. If bark is still intact when project is complete,call it caracter and leave it.

What klem1 said about using a router. YouTube has several videos showing them in use. Mine is a lot like this one...

Here is a link that might be useful: Router

Call around to local custom cabinet shops & ask them if they will run the slab through their planer and sander. There is one custom cabinet shop where I live that will do that for a fairly reasonable hourly rate charge.

If you can group your slabs so that they are doing multiples - the charge becomes more effective as the rate charge is spread over more pieces.

The shop I've used charges a minimum of one hour time, and it only takes them about 15 minutes to run one slab as they have a double sided planer and then two runs through the drum sander.

Thank you for the tips! I particularly like the idea of finding a shop that can run them through the planer, and I will look at making the router sled idea, my speakers are down but watched a bunch of videos and get the idea. Looks like I don't even need a bench top planer at this time.