I use and swear by big cheese rodents. The only problem is they sometimes run out of stock but you can easily view on their site if they have them in. The quality is top notch and packaging is really well done. I buy 3 trays of small rats at a time and 45 lasts my adult over 5 months.
They sell in trays of set amount per tray but you can get different sizes on the same order (per tray of course). Only issue I would see is your snake outgrowing a tray before being done with that size. If that isn't the case, rat fuzzies are 25 per tray.
If you are worried about quantity and your snake outgrowing them, I think perfect prey offers smaller quantities. However shipping would be expensive if you aren't buying in a bigger bulk.
Buying in bulk online saves so much over local pet shops but again if you are feeding a juvenile that keeps growing I could see it being a concern.
Check and see if a reptile show is coming up in your area as that is hands down the best place to get frozen rodents.
Either way if the size is only temporary, paying the little extra for a small volume from a local store might be the way to go until you get to the next size up and can have a larger amount for a while.