Tom's Party Store on Grand River Ave, headed toward Okemos, it'll be on the left side of GR Ave. They are nice store with decent assortment.
You could also try Big Ten Party Store JUST past the campus/downtown proper, on the right side of G.R. Ave going east toward Okemos, where the road becomes 5 joined lanes. It is a small storefront, next door to a gas station ( I forget which brand). They have another Big Ten branch on Kalamazoo Ave south of the Frandor Shopping Center, which is only maybe 2 1/2 miles from the other one-but west and south of campus.
Between these two, and you should be able to find whatever is available in East Lansing.