The problem with what you're looking for (organisms that are highly substrate specific) is that you won't get diversity - their specificity of environment just won't allow for a wide range of creatures to be housed. Instead, you should try to hit up as many generalists as you can to increase species diversity in your sandbed/clean up crew.
My suggestion would be to collect sand from as many different sources as you can to get the maximum number of varities of sand dwelling creatures that are easily housed/cared for in small closed systems.
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Spontaneously Purchased Scleractinian anonymous
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