Perfect Rhymes (rare): There aren't many perfect rhymes for "inseparable" in common usage.
Near Rhymes (similar sounds):
* Separable (this is the most obvious, but technically an antonym)
* Incomparable
* Irreparable
* Impregnable
* Changeable
* Unstable
* Indelible
Eye Rhymes (spelled similarly, but not pronounced the same):
* Separately
* Incomparable
Ultimately, the best rhyme will depend on the context and the desired effect.