Thanks for the suggestions, Tom. 
I originally considered using C3 and C4 plants, respectively, to create two
isotopically distinct diets but concerns were raised over the comparative
nutritional value of each (I was considering corn and potato). It seems that
there is a strong precedent for the use of corn in feeding experiments,
though; any sense as to whether I should worry about its relative lack of
nutritional value?

Thanks for your help,
Alyson