Check local building supply yards. Two-inch foam is a standard big box item in my area.

What thickness are you looking at?

2 inch rigid foam is in just about every big box and runs about R13.

No foam runs more than about R7 per inch.

I don't care about thickness, as long as R value is at least 13. For some reason, neither of the big boxes in my area (Massachusetts) had rigid foam with R values more than 6 or 7.

Must be a people Peoples Democratic Republic thing.

Foil faced 2 inch thick foam is commonly available.

Try a building supply house instead of the big box, or just use two layers (offset the joints and seal them well).

Something a lot of people are unaware of, box stores are home improvement centers glorified hardware stores ,not a supply house or lumber yard. You won't find a 2 x 12 x 20' at these places.There are six of these stores, all the same distance from my home,you would be surprised at the product diversity from store to store.

"Must be Peoples Democratic Republic thing".
Willard Romney aka Mitt Money was governor.
The Peoples Democratic Republic aka Cambridge, Ma. home of Harvard and MIT right next to Belmont a strong hold of the right.

"strong hold of the right."

Better tell Boston.

Did you look at the large sheets?

2 inch rigid insulation is usually in 4 foot x 8 foot sheets, not smaller panels.

And you can always just use two layers for a higher R value in a space.

If you go to a building supply house you should find a better selection of material, and even large sheets already scored for breaking down to standard widths.

Good idea to check supply houses. Aside, do 2 sheets of R-6.5 add up to R-13?

RK

"Aside, do 2 sheets of R-6.5 add up to R-13? "

Yes.

You could use two sheets and stagger the seams and be further ahead. The area between the two pieces would also allow another thermal block even if the two pieces were held together it would not be as tight as one solid piece.