Time of the year is certainly right - if there is a windy season there, it's now.
This said, the further South you go, the less wind you can expect during South West Monsoon. I have no idea if there is any rental equipment in Bentota.
Depending on how long you have for kiting, you could bring your stuff, and head North to Kandathoduwa, Alankuda or Kalpitiya before or after your trip.
If you have only little time, your best guess is to rent in Negombo. You fly in and out from Negombo airport anyway, and Negombo has better wind already than Bentota. There used to be a water sport center in Negombo that has kites for rent, but I forgot the name. Ask Hassan, the owner of the Firedragon Guesthouse in Negombo, for advise here, he is very well connected and a big water sports fan.