I am not sure how close you are to Amherst, but I trek yearly from the Cape to Andrews in Amherst. Great selection of perennials and annuals.

check out bay state perennial in south deerfield

I second Bay State Perennials in Whately. They aren't huge but have a nice selection of plants you don't find everywhere. Also, Wanczyk's in Hadley is worth a trip.

Third vote for Bay State

concur with Bay State, Wanczyk's, Andrews; would also add Hadley Garden Center now owned by Gardner's Supply (Vermont Company, employee owned)