As others have mentioned WiFi (802.11) and WiMAX (802.16) are different technologies and generally different devices are used to connect to networks of these types.
USB WiMAX subscriber devices are available for purchase though typically you are limited to purchasing these through channels established by your WiMAX service provider such as Clear or AT&T as Ricky has indicated. Purchasing through these channels is meant to ensure compatibility with the network deployed and maintained by that service provider. Note that WiMAX deployments by service providers may vary and a subscriber device meant for one provider and region may well not be able to even see the network of another provider or in a different region.
I have a laptop equipped with an Intel half mini PCI-e WiFi adapter which also includes a USB WiMAX subscriber device on the same card (the Intel ProSet Centrino Advanced-N + WiMAX 6250 half mini PCI-e card). This WiMAX subscriber station and the Windows drivers provided by the laptop vendor for it are pre-loaded with profiles to connect to various national WiMAX service providers through some collaboration between the hardware vendor (Intel), the OEM system integrator (Dell), and the WiMAX service providers. You may be able to find a USB device that combines WiMAX and WiFi like this but I have not seen one to date.
One additional consideration for you ff you are connecting to a closed WiMAX network is to be sure to coordinate your selection of WiMAX subscriber station device, which is the WiMAX equivalent of a WiFi adapter on your computer, with your selection of WiMAX base station, which is the WiMAX equivalent of a WiFi access point, to ensure compatibility based on operational frequency and various other WiMAX network parameters.