The R.Y.N.O is considered an illegal weapon in the Ratchet world and the regular vendors don't sell it. The only way to buy it is travel to Planet Rilgar and find the Shady Salesman. Speak to him and a cut scene will appear and he will speak of the R.Y.N.O and Clank will explain more about it. You then have the option of buying it off of him for 150,000 bolts.
There is no way you are getting that kind of cash early on. I recommend buying it on challenge mode as you won't have to grind credits as much. As soon as you buy it the trophy will unlock shortly afterwards. See Bolt Collector for money making tips.