Which version of a Bug's Life are you trying to find? The 2 disc collectors edition appears to still be in print, and is the best version to purchase. However, offhand I don't know the cheapest place to purchase it, but it is in stock at
Amazon and
DVDEmpire.
Since I live in California I can't really help you with the sales tax question. But the 2 stores I mentioned above are top notch, and you can add Deep Discount DVD and DigitalEyes to the list. I haven't purchased from DVDPlanet since the days they ran under the name of Ken Cranes. But would assume the problems you read about earlier are better now. They are owned by the same company as DigitalEyes and Deep Discount. Even though they are the same company each store is run differently, and prices can vary from store to store.
Also another good source for DVDs is through Columbia House DVD Club. I am sure you would have to pay sales tax in New York, but the money saved would far out weigh paying the tax.
DVDTalk is the best place to learn how to maximize your savings at CH. Unfortunately they don't have every DVD title available, or carry every studio like Buena Vista or Disney releases. Also they don't have any Criterion titles. But they do have a good selection from such studios as Warner Brothers, Fox, MGM, Columbia, and a few others.