2" x 3" Flexible Downspout Adapter
(or if you have a 4" round downspout, you can divert it directly into the barrel. Or you could cut an outline of your own downspout in the top of the barrel and use your existing material. This is more work and less clean than my setup, but cheaper I guess)
3/4" Brass Hose Bib
3/4"MHT x 3/4"MIP x 1/2"MP Brass Adapter
3/4" Black Iron 90 Degree Elbow
All of the materials can be found at a local hardware store, but if you are going to make a lot of them I would check at a large pluming store for discounts on bulk orders. I was able to get my costs way down this way.