I have heard of people using an iron with good success. Personally I use a Heat-Sealer from Uline that cost me about $95 delivered. The only drawback is that if you set the heat too high it burns out the metal piece that seals the bags. Set it at #4 and it will do fine; #3 setting will work for the five pound valve bags.
Uline is a great company to work with, excellent customer service.