I don't know that there is a "best" place to buy Disney tickets.
If you are really talking about the "cheapest" way to buy tickets, it would be with the Disney Club or AAA discounts which amount to about 5%. AAA tickets are available at your local office. DC discounts are available through CRO, at the Disney Stores, at any Disney Resort or at any park.
If you are not a member of either of these clubs, then then I would suggest TicketRES. THe discount there is slightly less than the DC and AAA discounts. You can purchase tickets there and you can have them sent to your resort or you can pick them up at WDW. To order tickets through TicketRES, just click on the link at the top of this page that says: Discount Park Passes.
I hope that this helps!
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