Your local builders merchants should give a good price... if it is a large order you need to bargain them down - I am not very good at bargaining, so my method is to go into BM A and ask for a written "best" quote (just say you are shopping around - make it clear you are not buying that day) then go into BM B and look for a "best" quote - if it is lower then go back to BM A with their quote, if not, show them the BM A quote and ask them to beat it. If they don't you know you have a good deal with BM A... keep cycling between them until you are happy with the final price. 10-15% is a good cut if the BM is reasonably priced to begin with. If you have a large order it will be delivered directly by Xtratherm/Kingspan.
There are seconds available in these products - but don't bother... most of the time it is useless... why bother with expensive insulation if it has huge air gap holes trough the centre of the sheets.