Welcome, Roarap!
I feel it far too difficult to produce a two-component polyurethane foam in your kitchen. But maybe you can find PU foam for cheaper than when it's sold specifically to archers? As insulating material to build houses, maybe it's cheaper? Though, the prices there aren't very encouraging
https://www.alibaba.com/showroom/high-density-polyurethane-sheet.htmlUsing a bought two-component PU resin to cast plates or slabs requires some learning but isn't very difficult. Though, I expect comercial slabs to be cheaper than the resin that a consumer could buy. To be checked.
Or could you use other materials?
Straw would be cheap.
Wood chips in a coarse hessian?
Recycle old newspapers? Crumpled, possibly wet then dried, and compressed to the desired density and shape? Once destroyed by your arrows, they can still be recycled as paper - unless you have to add some glue.
If one material suits the use, produce and sell the cheaper targets on a wide scale.