They're called geta, they're traditional wooden sandals, and they can be found online if you want a pair. I myself bought a pair about a year before naruto even came out. They are now dubbed my 'Jiraiya shoes.'

Sennin isn't really an official rank, but since Jiraiya, Tsunade and Orochimaru are so high in rank, it seems to be used as a non-official way to designate them as being considerably higher in ability than the average jounin. It really mostly means 'sage.'

I don't think that there's necessarily any connection to Spirited Away, besides the use of the same word. Jiraiya does not appear in the story.

I believe that Haku no Fuyu's answer is most likely the correct one:

The head protector he wears (as well as his costume and mannerisms) are all from the Kabuki play. Right down to the Geta.
Jiraiya is actually a character borrowed from traditional Japanese folklore, who was then made into the hero of a traditional Kabuki play.