Permanent interest bearing shares are issued by building societies and pay a fixed rate of interest. They cannot be sold back to the society but can be bought and sold on the stock exchange, which means the price varies.
Pibs are not risk-free. Though they are shares, they behave like bonds. If interest rates rise, their price falls. But if rates fall, capital values rise.
If your Pibs issuer gets into difficulty, you could lose money. The Financial Services Compensation Scheme does not cover them. That said, building societies are - generally - considered to be very low risk.
In terms of where to buy a PIB, you can do so through any stockbroker. There is generally no set investment minimum, but stockbrokers' dealing costs make investments of say, under £1,000-£1,500 uneconomic.
Editor, moneysupermarket.com