Before we begin, I have no personal experience with these vendors. I do not recommend, endorse or guarantee.
Level Zero
http://www.sieversbenchwork.com/You supply either a sheet of whatever, or add risers and sub-roadbed. Or both.
Heavy, expensive, but reliable and in the business for a long time
Level one
http://www.miannebenchwork.com/default.htmTim - the owner and a finish carpenter - makes the frame and legs but you have to supply the plywood, OSB, or extruded foam. Not very useful for a riser approach ('open grid'), but great if you just want to throw up a sheet of 1/2 inch plywood and move past the benchwork stage. Choose your depth (24" or 30" to the wall, if I recall) and you may get the home center or lumberyard to cut the wood for you.
If it were me, I'd use two levels of insulation foam, allowing more strength AND a place to cut in for wiring or scenery, bridges, culverts -- water features as they say. Lightweight compared to Sievers if you move a lot. Don't know this vendor's current status but we corresponded in 2010.
Level two
http://tomthorpecurvedbenchwork.com/2.htmlI expect these folks don't handle small jobs but I hope they would be nice enough to recommend someone for you
Level ur, um, hyperspace
http://woodlandscenics.woodlandscenics.com/show/category/ModURailSystemThis is a completely, shall we say 'original' approach. It's pricey but a few have written that it met their needs. You can choose the legs separately, the tabletop, the track pack (maybe), and a scenery kit. Not great for complex track plans. Try to find user reviews or published articles before plunging in. I'd say it's a 'love it / hate it' product