You should be aiming to feed your royal a prey item which is roughly 1x to 1.5x the widest part of the snake. Royals can be notoriously picky however, so some experimentation may be needed. Some will only feed of mice and not rats, some will only take smaller items, some will only eat rodents with colour on them and not completely white. Each snake is different and you should be familiar with and prepared for royal's eating habits and tendancy to fast when sexually mature.
They are great snakes and many feed with no problems, but you should be aware of possible issues. I often have disagreements with my two about when I think they should be eating and when they do!
Personally I would look locally for a supplier of food.. it can be much easier to buy small amounts of mice/rats in from a local shop, than to worry about bulk buying, shipping and storing large amounts - especially for a single snake.