Tax free PSU(Government) bonds are sold through official distributors and designated banks and post offices.One can visit any designated bank and submit the application form accordingly and once it is processed he/she will be getting a bond certificate in their name.
If one has demat and if bonds are listed on exchanges then one can also apply accordingly.
Through Zerodha one can not apply.
Generally people who invest in these bonds are likely to hold till maturity so secondary markets are very illiquid and difficult to exit without much penalty.
A retail investor can invest upto 10 lakhs in each issue.