Sometimes they are hard to find. Trader Joes stopped carrying them. I get them at my local market in the tofu area or in the refrigerated organics. They come in small plastic bags packed in liquid and must be refrigerated. The brand I use is "House" brand. Ask your market to order them. If you have Asian grocers by you they should also have them. They have the white ones (the ones I get) or gray ones which frighten me and I haven't tried. They carry no name brands and are much cheaper.
There is also a company that sells them on line. I haven't tried them and I assume they come dry.
http://www.miraclenoodle.com/Now, about the Shiritaki. I think they are wonderful but they take some getting use to and require special handling. They do not have the same texture or bite of a wheat based pasta. The best way it describe them is slippery. They don't break up in your mouth with chewing in the way you would expect but rather slide down your throat. Some people are turned off by the smell when you open the bag. I think they smell "fishy" so here's what I do. You got to rinse them. I soak them in cool water and swish them around and break them up a bit. I rinse them again. Then I put them back in a bowl of cool water with some vinegar added to the water and let them soak again. Swish, drain and rinse again a few times. Depending on how I plan to use them I vary the vinegar. Usually I use Rice Vinegar or just plain white vinegar. After you are sure the vinegar is rinsed off drain them well. I blot them with a paper towel to dry them. Then I microwave them for one minute. I then blot any moisture out again. They really taste better when they are dry...except of course when you put them in soup which is one of my favorite ways to eat them.
I can't tell you how much I love this product. As a dieter they have saved me on many "hungry" days. They make you feel like you are eating something substantial and I find them filling. A bag of the "House" Brand is 40 calories. So that's 20 calories a serving. Can't beat that for noodles.