I thought this was informative.
What's the correct way to pronounce 'Tesla'? We asked.
like the S in 'islam' and 'legislator' some people say it with a z, and some with an s. both are correct and reflect dialectal differences.
Nikola would have pronounced his surname with the unvoiced s (not z) sound in his native tongue. But there are tons of surnames with foreign origin that we mangle in English and still consider it proper/correct.
So in the end, it doesn't matter. Say it the way you like to say it.