you'll get a different answer from everybody you ask.
A good general rule is to feed items that weigh about 10% to 13% of the snake's weight. (I always split the difference and used 12% for my reference.)
So for example, if your snake is a baby and weighs about 15 grams, you should feed a Pinkie that weighs about 1.8 grams. If your snake weighs 80 grams, you'd feed a Hopper weighing about 9 grams, and if your snake was a full grown adult weighing about 300 grams, you'd feed an adult mouse weighing around 30 to 35 grams.
Most people will tell you at that point, that a 30 to 35 gram mouse is as big as you'll need to feed. As the snake ages it's growth rate slows and you'll go from weekly feedings to a schedule or 2 to 3 week intervals.
As far as the scale, I got mine at Target. Both Target and Walmart have nice scales in the "Kitchen Wares" department. It's actually a food scale and weighs in grams or ounces. You can find them starting at around 10 bucks, but you could give 40 or more for a nice one. I think mine was about $35.00.