Don’t now if this is the cheapest one, but you need it for the arduino pro mini 3.3v/5V to program: http://cgi.ebay.com/?ViewItem&item=321149890961
And for the 3.3v Battery projects you can drain you battery even more down with a 1-3.3v step up converter: http://cgi.ebay.com/?ViewItem&item=171569922743