I picked mine up on Ebay, last year we picked it up at the mall (they have this kios that just sells calendars in December) but here is what I've found for you;
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I didn't include places that I know are sold out, just the ones with the stock in at the moment. If you need more links or anything let me know.
Personally I would really look at the Amazon and Ebay pages, those are dependable and tend to be cheapest, the average price seems to be around $13 to $15 dollars USA.
Also, be really careful as they seem to use the same Scooby pic on the 2007 and 2009 calendars, remember to just double check it. ;F