Any type will do really, but mostly the round ones of 'pretty' colors (pink, orange, green,... ), or any you or your family have laying around.

See for example http://www.scrapbooknederland.com/assets/s2dmain.html?http://www.scrapbooknederland.com/10000096b60ce5193/index.html and (for the moment) http://www.scrapbooknederland.com/assets/s2dmain.html?http://www.scrapbooknederland.com/10000094ef0851a04/index_1.html

I have two of the 'grab bags', bubble gum and a mixed one. Others are very welcome!
I have most of the doodlebug buttons but more never hurts!