Depending on your scope of work, it may be better for you to bulk order the foam core.
I have seen some amazing deals on white foamcore in store, but the black typically runs 4.99 pre tax a 20x30 sheet around dallas. Hence my next foamcore purchase will be from the second link. Runs roughly 3.40 a sheet after shipping.

I just got back from Target just now. I got 5 sheets of black-on-black 20x30 for $3/sheet.

I'm also a noob here and have a question. I'm getting ready to do master boards for heckromunda/Rezolution (hence the trip to Target) and have purchased the Mars Station so far, but I don't want to use ground tiles with a grid pattern (which are my only choices with Mars for concrete)... so I'm looking at the Mayhem series, specifically either Bits of Mayhem or Industrial, as they both have weathered concrete ground tiles. Are they identical? Is one better than the other? I'm leaning towards Bits for all the extras like burnt-out cars and so on. Anyone with insider knowledge have an opinion? Thanks!

The foamcore I get at $1 a sheet on sale at Micheal's and the black on black I get at the hobby shop (bigger pieces and slightly cheaper than Micheal's) is all 3/16" and mostly Elmer's Brand.

What Outlaw said!

I've found both white and black-on-black foamcore at Office Depot, Office Max, Staples (all office supply stores) and Michaels (craft store). The best prices on white I ever saw was $1 a sheet (standard 20" x 30"). Don't remember what I pay for the black (comes in a two-pack for something like $8-9 though.

And both are 3/16" thick.

If it helps your search, the black-on-black two-sheet shrinkwrapped pack has a sticker with the following info:

Elmer's Foam Board
901-486
Black-on-Black
20" x 30" x 3/16" (508mm x 762mm x 5mm)
Barcode: 79946 04432

Distributed by Elmers Products Inc
www.sturdyboard.com

Hope that helps.

Steve