I was wondering that myself. When i had low humidity I put containers of water in the grow area with the fans blowing across them. That seemed to help a little. A humidifier would work if you had a whole room.
I had the same humidity you had. I bought a cheap small room humidifier from target for 16 dollars. It holds a gallon of water and I fill it every night. Keeps my humidity about 60% or so.