Now, I haven't personally been there, but enough of my friends have - and rave about it - for me to recommend the Cadeg Flower Market. Apparently it is where a lot of the flower vendors buy their stock, and the prices are fantastic and the selection and freshness can't be beat. It is recommended to visit there in the morning.
Click here to visit the official site.
Address: Rua Capitão Félix, 110 - Rio de Janeiro, RJ
By Metro: get off at Triagem station + bus 474
Flowers: Galpão (annex- Feira das Flores): Mon-Sat from 04:00am - noon; Sun closed.