You can complete the mandatory course anytime in the certification path. The only constraint is you won't get the success kit and the certificates unless until you attend this mandatory training and submit the details. For more details on this please check this thread.
Regarding the location of taking the training, you can always take the training online. You need to install some virtual software from oracle and you can attend online. They are called "Live Web Classes LWC, Live Virtual classes ". For more information check this
The list of training coerces can be found at this oracle website. You can select any one of them and click it. It will provide you the available training schedule for selected course.