It's really not a good idea to save your own potatoes from one crop, to use as starter spuds for the next year. You won't be able to tell whether they have any viruses, and by the time you find out you may well have infected other people's crops as well as your own.
Local garden centres may sell them loose; or any allotment associations nearby. Failing that, Wilkinson's sell them too :-)
P&P is generally high if you buy them online, so check that aspect out before deciding whether to buy local or from a bigger supplier.