I keep my vacuum packed ones for a year or more. Ones just regular Ziplock bagged 6 months max. They tend to dry out just like any of our food, pinks especially, and most of my snakes won't touch them. =P
To recommend an online source, I've had great success with Big Cheese Rodent Factory. They have small enough lots (starts at 50, I believe). They are a little more expensive than the other frozen providers, but the mice are healthy looking, clean, and they come vacuum packed. You might ask them about ordering different sizes as well and combining it to meet the minimum. They're great to deal with over e-mail.
Or there is a local guy who I get my mice (live breeders) from who may be able to sell smaller amounts than anyone else. His mice are great quality and I've had 100% feeding success. He doesn't have a webpage yet, but his name is Jeff Hermann and his e-mail is:
[email protected] Drop him a line and ask him what he can do for ya.
I suggest everyone invest in a Foodsaver vacuum packer. I use it for rodents, steaks, chicken, veggies, dough, anything in my freezer. =P
And a hint, get more snakes to make getting a larger number of mice more useful. It's an evil rationalization. >=)
Hope that helps some. =)