Even if it's a baby, your snake should be eating I'm going to say at least small adult mice or rat pinkies. That said, there are a lot of great places online to buy, but shipping costs are a huge factor and definitely something to consider. For me, shipping for a $50 order costs me $50 so I'm not yet at the point collection-wise where I'm going to start ordering F/T online.

A simple google search for 'frozen rats' brings a whole list of places to buy online.