I believe chinese birch is the stuff one can get at Home Depo for about $35 a sheet. I have found it adequate for builing forms. It does usually have some voids in the laminations. These can be easily filled with Bondo which hardens quickly or with 5 min. epoxy mixed with sawdust. The good stuff is called Baltic Birch and is harder to find, has no voids, more laminations, is flater and is considerably more costly. If you shop at home depot, look for damaged sheets which they've sold to me half price or 2 for the price of one.
If you're making your first form, a couple tips: drill pilot holes and countersinks, or the screws may force the plywood apart. use coated course thread screws and plenty of glue. leave one edge proud so you'll have a straight edge to square up the form. square up the form on all 4 sides so you'll have straight square edges to lay out from, unless you're using a blueprint, then it isn't necessary but won't hurt. MDF sucks.
John