It's hard to do this, I've wrestled with the ideas as well!
(If possible, try to attend a reptile expo within an hour or two drive from your location!)
I got lucky and had a reptile expo come to town- ( We usually have about 3 a year, so I stock up about every 4 months)
I got 50 big fuzzies, it only cost 42 bucksat the expo..... But the good thing is, about 30 of them are really big, and 20ish are smaller, so my 11mo snake can take the big ones, because he is sized for them, and my 8mo eats the smaller ones, and throughout the next couple of months the smaller one will probably move up in size anyway, so it worked out.
And if I have some left over because they've moved up in size, I just double up until the existing mice are gone and then go to an expo to get the next sizes up.
There is also
www.feedersource.com (They supply the expos as well, and I've never had any issues, the mice have always been healthy pink, clean and parasite-free)