Local pet stores that carry nightcrawlers will have them all year. You're best buy may lies in the hands of WalMart. In California, I get them for 3.25 +tax for 20 large there (compare to $4+tax for a dozen at local pet shops). Make sure you purchase the undyed nightcrawler at Walmart, though.
I am not sure why your frog lost its interest in crickets. Perhaps that was just a temporary thing? Or you frog had outgrown even the largest crickets around?