Look in the woman's aisle for volumizer shampoos. They have tons of them.
EDIT: As an aside, my new hairdresser said she has read studies that say using Tea Tree in shampoo actually helps reduce hair loss in men. She said the entire hair care industry is shifting to include tea tree oil in all the higher end men's hair products because of it. I have used Paul Mitchel's Tea Tree Shampoo and Tea Tree Conditioner for almost two decades now because I love the scent and the way it makes my scalp feel (I also used to have dandruff and it completely conquered it - I no longer have it at all). I also have almost all of my hair still, just a little thinning in the back, which is not bad for being 45 - especially since my dad and grandpa both had huge bald spots on the back by age 35.