Best material for inexpensive amp stands

Tvad's suggestion is great (as usual).
Another option is Bamboo cutting boards.
These come in many sizes (small to very large) and
can be found in the housewares section of major department stores and also Linen 'n Things.

i glued together 2 pieces of mdf and finished them off nicely with sanding sealer and krylon paint..

heavy brass cones for feet (you can get the screw in inserts from a woodworking shop (also of brass)

Little work on my part, but relatively nice looking and accoustically dead. You can go a little further by applying some sound deadening material to the bottom (out of sight). Something like Dynamat.