Are you looking to buy or board? My family lives in West Houston and I have a house in Katy as well. If you looking to buy land in Katy, either you have won the lottery or had a rich grandpa who loved you. Land is not cheap out that way due to all the housing developments.
Boarding: There are a ton of options in Katy and Cypress. Best bet is to search "Horse Boarding" on the Houston Craigslist. I cannot recommend anyone specific just do your research. Seems to be a lot of people out there with "Land" and having a hard time paying the bills so "how hard is it to get some one to pay me to use my barn" types out there.