Wood

what wood to buy:
You will be buying cabinate grade birch plywood 1/4 inch. Only one side needs to be good, but that side should be clear of seams, fuzzyness, nots, holes, ridges, or anything else that isn't just smooth wood. The only way to figure out what to get is to look at 10 or so pieces, looking at them at an angle, with your eye up close, as well as feeling them. Then you pick what you think is the best and it'll be fine.
You will be putting 2 1/2 inch borders around the sides of your image so you will add 5 inches to each dimension. So depending on how much wood you are buying, factor that extra length and width in. Monday and Tuesday I will cut wood for you, but after that it's up to you to find a way to do that. (I take food bribes)
Don't worry about the wood being warped. It doesn't matter as it goes through the press. But don't be shy and really look and feel a good number of boards before you pick one.

wood buying information (if you can add to this information please comment):

Amherst Woodworking & Supply
30 Industrial Park Dr, Northampton, MA
(413) 584-3003
http://maps.google.com/maps?f=q&hl=en&geocode=&q=lumber+amherst+ma&ie=UTF8&ll=42.427511,-72.522469&spn=0.060187,0.140762&z=13&iwloc=B

It was recommended as the best place to get our 1/4" Birch plywood.
I bought a sheet for $27 (though I may have been given a contractor discount)
and it was fairly easy to find a nice-ish sheet.

I went by Leader afterward and found their sheets cost $30.03 and were in much worse condition.

Leader Home Centers: Amherst MA
150 College St, Amherst, MA
(413) 253-3411
http://maps.google.com/maps?f=q&hl=en&geocode=&q=lumber+amherst+ma&ie=UTF8&ll=42.400136,-72.522469&spn=0.060213,0.140762&z=13&iwloc=A

Cowl's has 1/4" Birch for$ 33.60. The wood is better quality than Leader but not as nice as Amherst Lumber, but what's nice is they can deliver for free, provided you are flexible with your time line.
Cowls Building Supply Inc
125 Sunderland Rd, Amherst, MA
(413) 549-0001
http://maps.google.com/maps?f=q&hl=en&geocode=&q=lumber+amherst+ma&ie=UTF8&ll=42.427511,-72.522469&spn=0.060187,0.140762&z=13&iwloc=B

make sure to show up a good half hour BEFORE closing time especially if they need to help you.