I will usually buy about half as many rats as I need during breeding season, and just rotate them through the cages until they're eaten. My bank account loves this time of year, I only need about 1/4 of the rats I usually do.

In your case, this may take a little time, but try only thawing a one or two rats at a time, and then using tweezers, offer one to a snake at a time. If none eat, you're only out one rat, instead of several.

-Jen