US plug adapter

To answer your question, yes, try the main railway stations, Midi, Central, as well as many electronics shops etc. even large supermarkets that have electronics sections, and places like PhotoHall and Media Markt.

However:

Before you actually use the adapter, you do realize that most US appliances work on 110Volts whereas the European power system is 240Volts?

If it's for something like a phone charger or laptop power cable etc. make sure it can handle a 240v input without burning / exploding.

Most low power electronic devices, for example chargers, can handle the difference between the voltages (and frequencies) used in North America and those used in Europe, and they will typically be marked something like "Input: AC(or~)100-240V 50-60Hz". Such devices can safely be used with just an adapter.

Should an adapter not be enough, though, you can buy an adapter/converter at electronics stores. I'm still using one for an appliance I brought from the US 15 years ago!

MANY U.S. appliances and devices work with 110 AND 220/240...you MUST read the information on the device. If it is 110v only you will fry your device instantly. If you have devices that are 110 only that you must use here, you can purchase power inverters in Brussels, no big deal. Adapters and inverters/converters are available in many places.