Vouchers are very hard to come by. In the past perhaps posting on the community would get a voucher but this has not been the case for a number of years. The best routes to vouchers:  

1) Attend TDX or Dreamforce in person in San Francisco  

2) Complete and be a lucky winner in a Trailhead Quest that advertises a voucher as a prize (note you have to be a winner - completing the Quest does not guarantee a voucher)  

3) Be or work for a Salesforce partner - they get a very limited number of vouchers each year  

You can also keep an eye out for local community events that advertise vouchers as prizes for participation. Occasionally there are competitions on platforms like LinkedIn. In general it's hard to get vouchers