No to department stores
The department stores are not the place to buy electronics, although they have big displays and assertive sales staff, their range is limited, as is assistance level from sales staff and after sales service is best forgotten.
You will find scores of cellphone stores all over the city but you often find only the cheaper end of goods, older models, remanufactured phones or Chinese knock-offs. Your best bet, as mentioned above is to go to one of the big cellular service providers to buy your phone - shop around. Again you won't find a vast range, but you will find knowledgeable staff, technical assistance for repairs and servicing and their after sales service is as good as it gets in this country (take that in whatever way you like).
For computer stuff, go to one of the bigger specialist stores. You won't find a huge range of laptops, just a few brands like Dell, Acer, HP, Toshiba. Service and parts for other brands in practically non-existent and will mean sending/taking your laptop to the States. You will see whole-page ads in the local newspapers, particularly in the freebie dailies, but beware it's often "no-name" stuff with little or no back up. Your best shot would probably be one of the stores on Romulo Betancourt west of Churchill between Bella Vista and Cacicazgos, probably the best is CECOMSA who also have a very helpful service department.