Generally the rule of thumb is 10-15% of your BPs weight for a food item. It also depends on the weight of the snake. If its under 500g, then 10-15% every 3-4 days. Some people do every 5 days and some do every 7 days. But generally in that area. If you don't have a scale, then a food item that is about as wide or up to 1.5 times as wide as the widest part of your snake. As for being full, babies are always hungry. Dot, my little pied girl was eating every 3-4 days food items that were about 20% her weight and she still acted hungry all the time. She is now up to 433g and mellowing out on food. She still loves to eat but doesn't act like she hasn't eaten in 2 years anymore. If your snake is over 500g then all that goes out the window as you now just need to do a maintenance diet which is usually one small rat a week or two depending how big your snake is.

As for online rodent stores, I heard good things about Big Cheese Rodents. I personally just buy live and do cervical dislocation on my rats for my BCI and BP as I view it like a home cooked meal vs frozen dinner but that's just me