I can't find available seats in my eligibility period!

Two other options

Keep refreshing the Prometric calendar every 5 minutes for at least 4 hours for two consecutive days, you'll likely find a vacant seat.

Many people will reschedule with time and you can take their place immediately.

Also, I suggest you look at the possibility of relocating your center to a nearby country (if this is possible) and see if they have seats in your allocated eligibility period.

Haisook said:

OK, if I schedule a seat in a nearby country, will canceling be a problem in case I find a vacant seat in my country? I mean is canceling immediate or not? free or paid? any technical issues associated?

For instance, Can I cancel the appointment in the other country, and immediately schedule an appointment in my country?

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It's not easy and they may charge you fees. The fees varies from region to region.
Generally speaking you have to cancel the appointment that you made first and then try to reschedule in another center.
If you cancel before 2 weeks of the exam they don't take a fee but if when you reschedule they'll take some fees.
Again, it varies from region to region and you have to grab a phone and talk to Prometric directly if you are serious about this.

Have a look at an example here
http://www.usmle-forums.com/ecfmg-forum/405-tranfer-testing-region-india-us.html