moss i-sszlee
Visit Wookieepedia:
Mos Eisley (pronounced /mɑs aɪzliː/, moss izelee,
And here's a sound clip of the pronunciation, also from Wookieepedia.
To me it sounds more like 'moss i-sszlee'
As for how a native Tatooian pronounces it, listen to Watto which confirms it is 'moss i-sszleee'.
Just for some background, the same Wookieepedia site describes it as being:
located in a valley somewhere to the southeast of the Jundland Wastes[5] and roughly 80 kilometers north of Anchorhead, near the desert palace of Jabba the Hutt.
and Lando Calrissian is supposed to have said:
"Mos Eisley is known as the galaxy's armpit. I guess there are worse parts of the anatomy where we could be stuck."
It was mentioned in Episode II: Attack of the Clones and seen in A New Hope.
Nothing can compare with the real Mos Eisley though ;-)