Usually, when I need cusip #, or I have the cusip and want the associated symbol, I simply start with Google and a search of whatever info I have on a company.
For ISIN number the same.
For example. Google "Tesla ISIN number" results in following
For more extensive search, you may try this website (I just now found it, you have to register for free), but looks like you can search for ISIN numbers
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