RS (2017) guide doesn’t mention it but you CAN get Barik transit cards at location OTHER than at the RENFE or metro stations.
The Bilbao websites mentioned many locations are available but no specifics. I asked at the front desk at my hotel and went two blocks away to a tobac and got mine. I didn’t bother checking options at the airport (BIO) when we landed - where it would be helpful..
Be sure when you buy the card TO ADD VALUE TO IT. €3 is just for the card but nothing for trips. I was able to have the nice woman add the €5 minimum so it was ready the first time.
Speaking of using it the first time... make sure the Barik reader flashes “green.” I had a little trouble at first. I used the ticket machine at the tram stop to check it and next time had success.