@canuckrace; Outside of the circuit (both La Source as well as Les Combes entrance) you will find stands with merchandise (most of it is not official or at least 2-3 years old). On track you have the official stands from the teams (all next to each other) called F1 village, here you also can find a shop from the circuit itself (F1 village and circuit shop are behind Silver 2 grandstand). When you enter the circuit at Les Combes entrance there is an official F1 merchandise shop with stuff from the most popular teams.
My advise is to go to the F1 village, also they have some simulators and showcars from different teams and tyre change games etc.
If you are planning to buy clothes for yourself then make sure it fits you (it is no problem to try it before you buy it) because you can’t bring it back (unless you will attend Monza one week later…)