We bring the pismos home from the beach without any water on them. They can live for quite a while without water. We get seawater either from the beach at coronado, from the scripps pier (best since it's filtered), or wherever you have easy access to seawater. You'll probably want about 4 gallons of water for 10 clams.
Once home, find a cool place in the shade. I have a shady spot in my yard, the garage is also a good choice.
Put some of the water in a bucket or cooler, and add the clams carefully, as dropping clams onto each other can cause them to crack and die. You'll want to make sure they're completely covered with water.
After 12-24 hours, take the clams out of the water, pour off and save the clean water at the top, and dump any sand and poop. The clams will spit, be careful. :) When putting the water back in, it's a good idea to pour the water with force to get some oxygen into the water.
Make sure none of the clams have died. They will usually put their foot or siphon out, and will close up once you move them around. If you are worried about any clam, put it in a separate container, as a dying clam will foul the water and cause all the clams to die.
Repeat the water changes every 12-24 hours until you are ready to clean your clams. Waiting 3-4 days will give the clams enough time to spit out the sand and clean their digestive track. I often start cleaning them earlier, knowing they'll be just a bit dirtier.
When cleaning them, I like to take them out of the water for about 30 minutes to let them spit out what water they have in their system. Usually about 1/2 the clams will "clam up" and be very difficult to get a knife into. I usually just leave those for another day. They can also be frozen for 20 minutes or more, which will cause them to open slightly, and not be able to close up fast.