Since Sportzone closed at Chiado a year ago, there are no large sport shops in down-town Lisbon (Baixa). There is a large FNAC store for electronics, books, cameras etc at Chiado.
For ease of access by pubic transport, I recommend going to either the Centro Colombo shopping mall, Metro Colegio/Luz-Militar on the Metro Linha Azul (Blue Line) direction Amadora Este or the Centro Comercial Vasco da Gama shopping mall at Parque das Nações/Gare Oriente, accessible via the Linha Vermelha (Red Line) direction Aeroporto or by suburban train from Sete Rios, Entrecampos, Areeiro or Sta. Apolonia staions. .
At both these locations, there are large Sportszone.pt sport outlets, as well as worten.pt and fnac.pt outlets which offer electronics, mobile, books, etc etc.