i have been asked by at least a hundred (okay, not really, but it sure seems like that many) people about where we got all our buttons. Well, we collected them over the year, bought some at Walmart bought some through a favorite site of mine, ETSY.com You need about 500 buttons for a large tree.
The best place to buy styrafoam cones is at Michaels (with the coupon, of course).
The pins are the most expensive, so buy them at Walmart, enough to pin on 500 buttons. we like the large pearlized ones.
Whalaa...hours of fun. It will keep your children, or yourself, busy for a very long time.
As for the pom poms, for the garland, the dollar store is the cheapest place I've found.
Have fun crafting!