Plywood...especially of the quality you'd prefer for a layout surface...has risen in price drastically in the past year or so. To add to that, the price a single sheet of the size you're seeking will be another amazing premium cost...I'm sure any custom millwork claiming the ability to do so will have waste further adding to the cost.
You might find a custom millwork to fabricate a tongue-and-groove preassembled sheet...again, premium costs.
Save the $$ for all these premiums for other items you'll value more in the long run, IMHO. The table top will eventually be totally covered by track, scenery, structures, etc.,etc.. In fact, some layout builders will put another sheet down...Homasote...on top of the plywood to dampen sound. You'll be even harder pressed to find a single 9X5-1/2' sheet of that!
Undeterred?
Good luck.
KD
