My take is you have two choices and the second one is a bit convoluted

a) you need to buy in person, it looks like Hawaii is your best bet as it has the lowest of the three for state tax

b) If you can buy from out of state and can get it by ordering online, find a place that is shipping for free. If you want to further your savings, see if the company is also featured on one of those cashback referral sites like ebates (I can send you a referral to join, or you can find it on the conga for ebates in FT).
Why? Let's use ebates and ordering from J&R (NYC based). ebates gives 2.5% cashback for J&R orders. J&R offers free shipping and no tax on orders shipped out of NY state. Their prices are also the same as Apples.

One other thing. If you are eligible for an Education discount (students, teachers, faculty), you should also check out and ask about Education discounts, as some units give $100 off.