For XPS, the cheapest source is usually building-insulation board from a builders' merchant or DIY store (the pink/blue/green sheets) — laminate sheets with foam-safe glue to make thick blocks. For EPS, packaging suppliers, insulation wholesalers and even appliance/packaging offcuts are inexpensive, and you can often buy big billets directly from EPS moulders.
Buy by density and dimension rather than brand, and check it is plain polystyrene (not PU or a filled/fire-retardant grade that cuts poorly). Offcuts and 'seconds' are great for test cuts. If you need precise large billets repeatedly, a local foam fabricator will sell blocks cut to size — often cheaper than shipping bulky foam across the country.
Buy by density and dimension rather than brand, and check it is plain polystyrene (not PU or a filled/fire-retardant grade that cuts poorly). Offcuts and 'seconds' are great for test cuts. If you need precise large billets repeatedly, a local foam fabricator will sell blocks cut to size — often cheaper than shipping bulky foam across the country.