Hi Leigh and welcome to TONMO.com!
I think the recommended amount is about 10% a week, or 20% every couple of weeks. Watch your water parameters, too - water changes are one of the ways to lower nitrates.
We've located large jugs of R/O water at Walmart's, Culligen's, Whole Foods, and many other stores. It's purified drinking water... just look at the label carefully, it will say "reverse osmosis" A five gallon jug of this R/O water would make enough saltwater for a 12% water change. You can also use distilled water as a basis for making salt water. I use distilled water for my top off water as the water level evaporates.
You could put the clams in a little net breeding container at the top of your tank - remove the any that are dead. What is this "shrimp thing" that is getting old?
Nancy