Have a look in AKI
C. Com. Colombo, Lj. A003
Av. Lusíada
1500-392 Lisboa
Tel. +351 217 110 750
They also have shops in Alverca, Cascais, Telheiras, Expo amongst other.
See: http://www.aki.pt
Also try Leroy Merlin
Estrada Nacional 249/4, Km 0.1
2710-088 SINTRA
PORTUGAL
Tel: (+351) 21 925 37 00
See: http://www.leroymerlin.pt/lmpt/bricolage/lojas/Sintra/sintra.html?page=5
Also along R. São José that runs parallel to Avda. da Liberdade, there are some electrical retailers who may have what you want or who may be able to point you further in the right direction.
