Green's citations, beginning in 1935 include Clouds Seven, Eight
and Ten with the same meaning. Earliest attestation of "9" is
in the 1959 pop song "Kookie, Kookie, lend me your comb"; and
OED has one from Down Beat (jazz magazine) in the same year.
(His etymology mentions an improbable connection to a classification
of clouds by the US Weather Bureau. OED more plausibly connects it
with "in the clouds" (from 1651) "obscure, mystical; fanciful, unreal;
above the range of ordinary understanding (generally combining the notions
of obscurity and elevation)". There's a cross-ref. to "seventh heaven",
but oddly not to "head in the clouds" (1806-) meaning "to be detached from earthly matters; to be out of touch with reality; to be dreamy, impractical,
or unworldly". It was a trope just waiting for drug fiends to get their
hands on it.