A couple options for you:
Layne Labs has very good quality rodents, they are who I order through consistently. They allow you to order smaller amounts than most other large rodent suppliers.
They are on the west coast (CA) so shipping will be more, but it might be worth it rather than ordering more than you can use.
Another option is finding someone relatively close to you, who will go in on an order with you. That will help the cost.
Each animal grows at a different rate.
A few years ago, I raised up a few babies from the same clutch, and they grew at different rates, on the same exact feeding schedule.
It is hard to say how many of each food size to get.
I do not keep rodents past 6 months, I usually go through them in about 2-3 months, per order. Your snake may or may not reach the size to eat hoppers before then.
Another option is to get more snakes.