Best foam?

Hey guys! New to modeling my own scenery and was wondering where you guys get your foam? I'm looking for 2" thickness and something I can use the ballpoint pen technique to create cobblestones/woodwork etc etc. I saw some Insulation foam over at Home Depot but it looked like it was a bit too, crumbly maybe?

Thanks!

Home depot is as good a place as any. Lowes and Menards carry similar brands. Home Depot carries Owens Corning FOAMULAR which is an excellent material for scenery building. Extruded Polystyrene Insulation usually comes in pink, blue or greenish 4x8 sheets (now in purple as well). Products with a higher R value will be denser. Owens Corning products used to be pink, hence the Pink Panther as a logo.

To avoid crumbly foam avoid the color white.

Personally I never buy the stuff, I just scavenge construction sites and theaters. Tons of it is thrown away every week in those industries. Just go by and ask if they are throwing any away. Not asking is generally frowned upon.

Von Kurst covered that nicely. Look at the R Value of the foam; the higher the number, the denser the foam is. This varies according due to thickness, but a 3 to 5 for 1/2" to 1' is good.
Also check out the Fan Foam...it looks like an accordion or fan. It comes in 2' X 4' sections joined at one 4' side either directly or by the protective film that covers the board. It comes in 1/8" to 1/4" thickness; ergo GREAT stuff for making buildings, ruins, et al. Lowe's own brand uses the film connection which makes it more readily usable.  

Awesome! Thanks guys.