Chances of you finding a summer litter are fairly slim. 90% of Basenjis typically come in season one a year and that is in Sept/Oct, so pups are usually born Nov/Dec. If you go to www.basenji.org and then to the breeder directory, you can find breeders in the areas you are looking or they can refer you to other breeders. Someone might be planning a spring/summer litter if they happen to have a bitch that has more then one season a year.
Can you tell us more about yourself and your current basenjis? Like Tanza said finding a summer litter are slim, but there are a few out there.
Would love to know more about your current pack, maybe even some pictures. We always love to see pictures of basenjis. And were you got them from as many of our pups are related on one way or another.