Did you think about upholstery stores?
I saw some rigid blocks in there, but even the soft foam is easily made rigid and workable soaking with really thinned epoxy (or varnishes/paint) and wait for it to dry. (actually its a great way to create a mold around exhisting objects draping it and then soaking, even using spray paints stuff works to make a thin layer rigid, of course protecting the object first)

Arts and crafts stores also carry some blocks but they might be too small

Companies that make signs use a very dense foam that could be ok for you, check their suppliers, no clue about the prices there though.

What about florist foam (the green stuff)? (not sure if thats PU)

Depending on whats the project there could be lots of materials you could use instead that can be more available or just cheaper.

: I'm looking for sources for rigid polyurethane (or comparable) foam for
: prototype/model/plug work - that will sell small quantities (like 1/4
: sheets where shipping won't kill me - or located in South Fl where I can
: pick up). 3/4" and maybe 1 1/2" thicknesses - 6 lb or maybe 10
: lb density.

: I know that somewhere, probably locally given the marine industry, people
: have what I need as scrap! Finding them is another matter. Dealers I'm
: finding aren't really catering to small model/prototype order.