Can only tell you where I have frequented before though some time back.
You know this area on Haille selassie av.where Margaret Wanjirus church stands?
The opposite side of the church has many agro shops selling all manner of seeds in wholesale plus lots of varieties as well.
Their prices are very fair cz you can negotiate depending on the quantities.
And yes you will find all the seeds you want to get you started from nursery.
"The true measure of a man is how he treats someone who can do him absolutely no good.