There are plenty of Rose / Flower Farms in Naivasha.
Both not sure if you're allowed to visit them.
Best to enquire from some of local hotels/lodges along Moi South Lake Rd in Naivasha.
Naivasha is a little too far if you are just interested in roses/ flowers. There are several large scale flower farms along the Nairobi, not too far from the city along -Namanga road (way to Amboseli). You can have a look at this one and probably write to them and ask if they admit guests- http://www.sianroses.co.ke
there are several a 45 min drive from nairobi