seconds quality kingspan is available, try secondsandco.co.uk but be aware:
They do pallet loads of boards that are priced based on a guy with a clipboard scratching his arse and saying "ooh, that looks like a big one, 200 quid" so if youre looking in the pallets section do work out if youre getting a bargain cos some of the stuff is more than first class kingspan
be aware that some seconds stuff is seconds because it's bent or the edges didn't expand properly so it's not always a uniform size. If I were you I would also nip onto CFS's website and get some 2 part expanding foam. mix it in small quantities and paint the board edges with it as you put it together. aso mix it in disposable drinks cups because it'll wreck every container you leave even the smallest residue in (I've got loads of blocks of kingspan around site all shaped like 2 litre pop bottles
). It expands a lot more and pushes its way into gaps a lot better than the canned stuff, but if you go for cans, probably the cheapest source I've found so far is a co called platinum supplies on ebay.. in the region fo £2.50 a can (plus vat) for the gunnable stuff. get a gun if youre going to be doing a lot
planetinsulation does reasonable discounts on orders over £2000 of first class kingspan, I think
in terms of what is good to walk on, most of my stuff has come from seconds, I've had recticel, kingspan, st gobain, silcart, silatherm, econotherm and loads of noname stuff. it's been coated in everything from tarry sand, paper, to fibreglass. would I walk on it without an overboard? sure, but I wouldn't stamp my feet. 9mm osb should be fine, even 5mm hardboard would probably do. the only one I'd hesitate with actually, is celotex, possibly also kingspan. the celotex is much less rigid than the other foams, and cracks/powders up. same for the particular kind of kingspan I had but it was only 25mm sheets. it was pink, and smelled like horse ****, which is odd because usually PUR/PIR smells like fish guts. It also powdered when stepped on