Size is important when it comes to flattening and smoothing your wood.
IsGood wWoodworks has a Grizzly 20” helical head planer and a 18” wide belt sander - so what if you need bigger?
Buy us a new planer/sander - warning they are not cheap ;-) But we will name the tool in your honour!
We have a router sled setup that you can use but its time consuming and messy
There’s a few places around the area that have wider sanders (wide planers are pretty rare but the sanders can flatten with low grit paper) or big CNCs - they charge by the hour.
Noble Woodworks in Ballard has a 52” sander
Makerologist in SODO has a 37” sander and a CNC that can level 5’ x 15’
AK Designs in Tacoma has a 4’x8’ CNC machine
Check out MakerBook.io where you can search for other places.