I got mine on Ebay from a company called Geetac. They dont do singles so I bought three which actually works out quite well as I always have spare filled nets. They have big holes, not huge but not like horsehage nets and are 4 foot by 4 foot! My ID loves them!
A word of warning though, I haven't used one since finding my boy with his front leg hooked up high in the large holes. Normally can be trusted with anything and he wasn't panicking, just waiting to be rescued
So far (tempting fate here) my ID hasnt got a foot in one. The holes are about 4 inch I think. He has only gone onto nets this winter as the price of hay had gone too high for me to waste so much by feeding on the floor. He does not approve and will kick at a net in frustration. Recently he has found a new trick, hooking the front strap buckle of his stable rug to the net and then pulling back so breaking the string! Im finding electrical insulation tape works well to cover the buckles and foil his plan!
If he wasnt such a big lad I would try a haybar but I can imagine it would also become the target of a 'game'!
You could always try making your own? My friend makes her own out of baling twine from the big straw bales, id always admired them & she made me 2 for Xmas...was well chuffed