We use PB8 (you can find locally) and we use Immune Support from Apex.
http://www.worldlifesupplements.com/products_main.html
We also do not eat sugar or sugar substitutes, are in bed by 7:30 for the kids (10pm for us), very little dairy (as it causes inceased mucus) and we use therapuetic dosages of vitamin A (which many people will grill me for the dosages, but might not understand vitamin A and it's true toxicity issues).
I give my kids 1/2 PB8 each night, as well as Nordic Naturals DHA Junior, 1 teaspoon of Bluebonnet CalMag and 1 A&D every 2-3 nights each week. The A is in 25,000iu and the D with it is 1,000iu for each capsule. Vitamin A is the primary nutrient for epithelial tissue, which is what the respiratory system is made of. if we start to have symptoms, I start the A&D before anything really starts and I get us all adjusted by our chiropractor. I'll give the kids 3-5 of them for a few days and then I'll take 6 or 7 of them for a few days and then maybe 1 for a week. Too much A will give a bloody nose....but once stopped ALL toxicity issues go away. (Retinal palmatiate is what causes problems with "vitamin A" toxicity in pregnancy, not acutal vitamin A.)
http://icpa4kids.org/Find-a-Chiropractor/
http://www.merckmanuals.com/professional/sec01/ch004/ch00...
We also use Herbs for Kids Echinacea/Goldenroot or Echinacea/Astragulus for bacterial issues and Oscillococcinum for viral issues.
My kids haven't even had so much as a runny nose in over a year, although they are around sick kids and adults constantly.