Hi flieaway, and welcome to TCS!
I'm so sorry you have to leave your kitties for the summer. I don't know how many members we have in NH at TCS, but I do have some suggestions for you. You may have taken these steps already, but just in case....
Place an ad in a local paper.
Does your college/university have a paper? Try that too.
Put up "posters" at local vets and local pet stores, grooming places, etc. Spend a little time - talk to the receptionists at the animal hospitals. Vet techs and receptionists there are notorious animal lovers. Most receptionists of animal hospitals around us actually pet sit (meaning they visit your home when you're away and feed your animals, clean the litterboxes, make sure they have water, etc. All of them (around here) that pet sit will visit your home one to three times a day and charge per visit).
No matter what happens, if you do not know the person (in advance) who is caring for your animals, ESPECIALLY if you are having the animals live in their home, not yours, please, please, please use an agreement, and have arrangements set up with a vet in advance.
There is an adoption agreement up on www.SaveSamoa.org
in the rescue section (in Word format), so it can pretty easily be changed into a Foster Agreement. You want to make sure you're getting your kitties back, that they're YOURS, and that they get proper care while you're gone. Provide feeding instructions, rules about not letting them out, emergency contact info, phone number of your vet (or if you don't have one, a vet you contact now - or a vet you contact near the person who will be caring them). Make sure they provide proper "tools" for your cats - a scratching post, that they'll keep the litterboxes clean, etc. And if the person lives in an apartment and rents, make sure their lease agreement allows cats. You don't want someone to foster your cats and three weeks into it have to "do something with them" because their landlord is threatening to kick them out....
Also, especially if the person has not fostered cats before, please make sure they know what to expect. Cats are territorial, being separated from you and the territory they consider to be theirs - in fact, a totally new environment, is going to be terrifying and stressful for them (at first).
Perhaps provide them with information to help them understand what is going to happen. This is a good link:Bringing Home a New Cat
Make sure they know how to work with cats. This is a good piece to provide:Cats and Discipline Don't Mix
This piece doesn't say it, but the best way to say "no" to a cat is with a little puff of air in the face. They generally learn from this VERY quickly!
Make sure they know what to look for in terms of illness. Kitties will likely have diarrhea the first few days just from the stress. If it lasts beyond that, they need to see the vet. If they don't use the litterbox, they need to see the vet. Etc.
I'm sorry I'm not writing in to say "we can do it!" - but I hope this has been of some help.
Good luck! ...and have a great summer.