When purchasing a curd knife you have two basic options – a roasting or carving style knife or a large frosting knife. The names are pretty self-explanatory, but to give you a better image: if you’ve ever been to a restaurant or buffet with a carving table, you’ve seen the exact kind of knife I’m talking about. I haven’t personally frosted a wedding cake, but I imagine an industrial size frosting knife is what most bakers use (see below).
I prefer the roasting knife because it’s sharpened. The edge makes it easier to cut your curds and it can be called into service to cut roasts if needed (I know I cut meat more often than I frost behemoth cakes). Just be careful – they’re sharp! You may want to purchase a paper sleeve to protect you and your knife. Just make sure you get one with a straight, not serrated, edge. If you opt for the frosting knife, make sure to buy one that has a straight handle (some, though usually only home kitchen sized ones, have an offset handle for ease of use).
With either option I recommend purchasing one that is a at least 10 inches long. As with most cheese making supplies, bigger is better! To make cheese in 2 gallon or more increments you nee a large and deep pot, and the curd knife needs to reach the bottom of the pot without dragging your knuckles through the milk. For reference, I have a 12 quart pot and a 12 inch knife works nicely with it.
The best place to get either of these is a specialty shop or a restaurant supply store. If you’re placing an order for cultures or other cheese making supplies you can probably just add a curd knife to you’re order. However, for the best deal, and a chance to test the feel of the knife, a drive to your local restaurant supply can be worth your time. There are a number of other useful cheese making items you can pick up while you’re there! Of course there are plenty of online restaurant suppliers, or if you’re lucky you may have a local store that stocks quality cheese supplies.