We had good experience in this testing centre which was efficient and cheap (think 25 Euro ).
https://democratest.com/test-antigenos/?gclid=EAIaIQobChMIk42A5tKK9wIVhBTUAR1FCwp0EAAYAiAAEgI1VfD_BwE These are their most central locations:
Clínica Veneer´s, Ronda de Sant Pau, 34-36 (we used this one, is a bit difficult to recognize as it is in a dental centre and is close to Las Ramblas)
Cínica C.E.M, Plaza de Dr Letamendi 34,
Clínica Arisa, Paseo de Picasso, 36