Indexes and indices are both accepted and widely used plurals of the noun index. Both appear throughout the English-speaking world, but indices prevails in varieties of English from outside North America, while indexes is more common in American and Canadian English. Meanwhile, indices is generally preferred in mathematical, financial, and technical contexts, while indexes is relatively common in general usage.
Neither form is wrong. Both have been in English many centuries (and though indexes is now most common in American English, it predates the United States by centuries). It’s true that indices is the plural of index in Latin, but index is an English word when English speakers use it—and it is a longstanding one at that—so we can pluralize it according to the conventions of English.
A quick tour around the world’s stock indexes will reveal the extent of Mr. Market’s New Year rethink. [Wall Street Journal]
This year is looking tougher, though. Asian stock indexes are off to a rocky start with increasing concerns over rising inflation and other macroeconomic conditions. [New York Times]
Indexes in idle mode: Stock rally fed by Europe deal slows [USA Today]
Despite the fact we have major challenges of poverty, in all the indices of community engagement we are thriving. [Guardian]
Of the 45 indices that make up the MSCI World Index, 36 recorded declines of more than 20 per cent. [Irish Times]
For the average direct investor or trader, the market and the indices that represent it are almost completely irrelevant. [Sydney Morning Herald]