I think (in the UK) the blue foam is XPS and tends to be used for flooring, so the cheapest source is the supplier/builders merchant. Google XPS floor insulation and you can find quite a few suppliers of pink as well as blue 1200mm x 600mm boards.
Worth noting that floor insulation is frequently thinner (5-10mm) and therefore cheaper than the thicker cavity wall boards, and you can source floor insulation that has higher compressive strengths -
slightly less damage from dings, dents and elbows. Probably worth a trip to your local Wickes/Jewsons/Travis Perkins to see what they have and its what you want.
You can buy blue 'craft foam' from this lot, as well as a wide range of other types -
https://www.panelsystems.co.uk.
Excellent service and pre-cut into possibly more useful sizes (as opposed to a 1.2m x 2.4m sheet) but obviously the craft version is more expensive than straight insulation.
Word of warning, I personally found it almost impossible to cut large sheets at home and end up with flush edges; boxing them in with wood would take me ages and cost a lot more than I planned, so I ened up with enough hills and cliffs to make the Himalayas - not quite what I set out to do.....