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This is an interesting question. For a power series f, the method f.padded_list() returns a list of coefficients, and you can return a single coefficient using list notation: f[k]. How are you representing the infinite product in sage though? If you're just multiplying a (large) finite number of polynomials or power series, then the methods above will probably do what you need; but maybe you're asking something more subtle?

Note: here is the documentation for power series.