Once you find a supplier, you need to be prepared to transport your material. You can put dry ice in a paper bag and place it in a cooler or other insulated container. For liquid nitrogen, you want an insulated container with a lid that doesn't make an air-tight seal. Some companies will sell you a dewar or special thermos made just for this purpose. Another idea is to use a big insulated plastic coffee mug. The mug is nice because it has a small hole at the top to prevent pressure buildup of vaporizing nitrogen, plus it has the advantage of being inexpensive. You can get about as much dry ice as you think you will need, because it doesn't sublime too quickly when insulated. Get a bit more liquid nitrogen than you will need for your project, because it evaporates rapidly.