I buy from FindTape.com. I prefer their ATG tape to the Scotch brand. If you buy 12 rolls (which sounds like a lot but really isn't!), it's $1.42 a roll. It's acid-free, but still a fantastic price when compared to the Scotch brand.
Shipping's a little steep, but levels off after about $8. I bought 24 rolls of ATG and shipping was $8.50. I added 3 CASES of hockey tape (very heavy cloth tape), and shipping went up less than $2.
http://www.findtape.com/shop/product...600&height=776