Android market is not available except on Google approved models. Most of the cheaper Tablets do not qualify as they don't have the features that Google requires. However, that are lots of other market apps available that you can use. Sometimes a third party will create their own Android OS implementation that can be loaded on a device and this may or may not include a version of Android Market. Note that the port will have to be specific for this device, and I am not aware of any at this time.
See our wiki under Android for a list of available market apps.