I searched the archive and found nothing, yet remian doubtful that this question has not, in some way been asked before…Why is there a pattern in respect of numeric monikers for paradise? Are there other less popular versions? What, for example is going on in 6th Heaven and upon Cloud 8? Anyone?
Aren’t there traditonal rings, like levels of hell in Dante’s Inferno?
The cloud thing is Hindu, isn’t it? Maybe it counts off the number of arms of one of those elephant-head gods.
Seventh Heaven comes from the the Islamic concept of a seven-tiered heaven.
Cloud Nine appears to be the invention of a radio show writer. The Johnny Dollar radio show of the 50’s had a recurring gag in which Johnny gets knocked cold; when he revives, he babbles on about the bliss he just experienced “on Cloud Nine.”
For seventh heaven, m-w.com gives the following:
No etymology is given for cloud nine, but it dates it at 1959.
Looks like stuyguy beat me to it. 
Seventh Heaven (the concept,not the phrase) predates Islam by quite a bit. In the intertestamental works by Jewish writers, there are references to the different heavens or levels of heaven. Seven, of course, is the perfect number, so that was the highest number reached when discussing them. The works that mentioned these heavens were usually apocalyptic in nature, so numerology and symbolism figured very dramatically in their descriptions.
Judaism let go of those ideas with the philosophical retrenchment following the fall of Jerusalem. Christianity never made a big deal about that (although there are early references to the various (numbered) heavens), but the idea probably hung aroung the Middle East until adopted by Islam.