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Avios points are typically linked to British Airways and its partners in the OneWorld alliance, which includes Iberia.
So if you have any Capital One Points or AMEX Membership Rewards points, you can transfer them to BA, and then link them to Iberia.
If you don’t have any points, then you can find a great offer for British Airways card or Iberia card that might give you, for example, 85,000 sign-up points.
That is how I got started.