Foam insulation under concrete slab

[TobyB]

Ok, be kind, I'm new at this-

where do I go, as a DIYer, to get foam insulation to put under slab- on- grade for a shop?
What grade of foam do I need for what's effectively going to be used as a garage floor?

The backstory is, we're having a barn built to use as storage and work space, and it
seems shortsighted to NOT put in the tubing for hydronic heating. No immediate heating
plans, (as in, this is a 'storage' building until the permit's signed off, and probably for a while after)
but we have room for a geothermal field and also lots of maples that are going to eventually
be removed...

Thanks!

t