Most Garden Nurseries and plant catalogues like DT Browns. , Thompson and Morgan sell crowns of Rhubarb.
But once you have planted the crown of Rhubarb, leave it and you will NOT be able to pick any of the stems for ONE year .After that first year you will have plenty of rhubarb to eat. The leaves you can actually put in the compost bin, despite what some people might say.

For good rhubarb, dig a wheelbarrow full of good rotted compost into the planting hole, something like well rotted horse manure would be good, or just garden compost.
Rhubarb does best in moist (not waterlogged) site and with rich soil will go on cropping for several years before you need to divide it. After three or four years you should dig up the old crown and use small offsets to start new plants as the old root becomes very woody and unproductive.
Good luck! There's nothing better than forced rhubarb in early spring or late winter when you can force the crowns by covering them over with a black plastic dustbin and some compost for a few weeks.