If you live in a samphire growing area such as the East Coast of England, there is no point in attempting to grow your own as it is so widespread. You simply need to don your wellingtons and make a trip to pick as much as you like. Do be careful of tides though and take expert advice.
If you don't live near a samphire growing area though and want to try some, you could try to grow some although given the very specific conditions in whch they grow, there is no guarantee of success.
You can buy a samphire plant and plant it into compost in pots in sunny conditions and water it with a solution of one teaspoon of sea salt (it MUST be see salt not table salt) in a pint of water. You will also need to ensure that the area is protected during the winter frosts especially.
If you would like to try samphire and your local supermarket or fishmongers does not have it, try the item at the top of the Amazon link on the right for home delivered samphire and enjoy eating it.