There is a tall white one called Mount Everest which would contrast nicely with the purple; for something a little more unusual there is bulgaricum(nectaroscordum siculum) which have flowers that hang in loose umbels of white, green and purple, or allium "hair" which is quite bizaar looking. Assume you are in the US so can't help you with source as I am Canadian. I love alliums and hope to add a few new varieties this fall; I am more zone challenged than you so have less of a choice!