Thrive Renewables (Buchan) 2016 Bonds can only be bought when a party that already owns some is willing to sell them or when the business does a new share issue.
Currently, Thrive Renewables (Buchan) 2016 Bonds are only available to purchase from existing holders via the Bulletin Board, subject to availability.
Transferable bonds can be sold on to third parties - provided willing buyers can be found. They also vary in value, subject to the price that and sellers want to buy and sell at. Bonds can be sold via the Bulletin Board.
You can find a guide to the process of buying and selling bonds on the secondary market here.