Find any woodworking shop that deals in furniture grade wood products as they will be able to order you any amount of sheets G2S (Good two Side), in any thickness, with whatever core you wish (spruce, marine grade), from their veneer supplier. You may also be able to specify rift oak, quarter, flat cut or rotary and have the sheets number matched with the same flitch of veneer. This is especially nice with rift or flat cut veneers.
You can even get veneer laid up on accoustic panelling.
You will pay for this, but it will be worth it.
Here is the company we dealt with in Ontario. Show this link to any wood working shop to find a comparable company in your area.
http://precisionveneer.com
Cheers
Tony