soil mix for square foot boxes

Perlite and horticultural vermiculite are more or less interchangeable for most horticultural purposes. Note that horticultural vermiculite absorbs water very well while vermiculite for insulation is treated to resist water. Few places seem to carry larger bags of either as they are used mostly for mixing your own growing media while garden centers would rather sell premixed stuff at higher prices. Perlite used as a lightweight aggregate in plaster or interior concrete is the same as used in gardening.

The four cubic foot bag of perlite that I am working on was purchased at Sid's Greenhouse in Palos Hills several years ago. Most real greenhouses use a fair amount although polystyrene pellets have replaced perlite in many bedding plant growing operations. Unfortunately, most local greenhouses don't really grow a lot of their own plants any more.

Edit to add: Perlite and vermiculite dust can be nasty, so wear one of those paper dust masks when dumping or mixing.