If stern rail seats are what you really want I suppose there are a number of mnufacturers that can provide the necessary hardware.
Before you go to the expense and trouble of adding these you should know that they are only useable when the boat is "parked" or motoring, in other words not under sail! It is very uncomfortable while heeled over to sit or stay in these seats. The top of the pushpit digs into your ribs, not comfortable at all.
Some of the builders include these cute things and people sit on them when at boat shows but they are not very functional while under way.
My advise, take the money and buy a high quality water proof hand held VHF. A friend of mine has them on his boat.....he refers to them as the puke seats!