You're looking for a restaurant supply item ( it's what they use behind the bar and behind the cook's line ). Costco carries them at times also, or check out flooring and carpet stores. They are not cheap, but very durable and come in a variety of thicknesses. Check out restaurant supply stores both new and resale, or check the classifieds for used restaurant equipment.
I got one for my sled at Aramark in NE Portland. They are out by the airport. Aramark is a industrial laundry that supplies shop towels,uniforms, etc. Give them a call to see if they sale used mats. Make sure you set something heavy on the mat when your trailering your boat because they can blow out, I know from experience.
Home Depot or Lowes in Hillsboro has what you want. Lowes has them for 2 bucks cheaper than HD. They are called anti fatigue mats. 3x3 for 17 bucks. Also the same stuff Motion Marine uses for sinker holders.
Most of the local Costcos are carrying the heavy rubber mats used in food service. They have 1 inch holes and knobby bumps on the bottom. they are very easy to hose off and won't fly out at freeway speeds. They are about$27.00 for a 3'x4' section. They are virtually indestructable.