The first x-302 was built at the very beginning of season 6, the premier actually named Redemption. That one was lost, on purpose I might add. The x-303 was also introduced in the middle of season 6, in the episode Prometheus, episode 11. In the same episode, Major Davis states that they don't have any of the 302s ready for flight yet so Teal'c and O'Neill have to use one of the death gliders they got from Anubis's ship that was crash just south of Alaska in episode 3 of season 6, Descent. So, what this boils down to is that they have had well over a year and a half to build new 302s. The ones we see in the season 7 finale, The Lost City I'd imagine. Also, the Prometheus, or x-303, had its weapon systems and shields upgraded with Asgard technology as a way of saying thank you for saving their butts again. This was mentioned in the episode Disclosure, when the Stagate project was disclosed to the French, Chinese and the English. Disclosure was a clips episode BTW.