Get all the seed catalogues - Thomson and Morgan, Kings, DTBrown, Organic gardening Cat, Seeds of ...(something, forgotten), Mr Fothergill, Marshall's, oh there are lots and lots, some specialist, some more general which they will mostly send out free - easier to look at than online. DTBrown and Mr F are good for prices and quality too, T&M tend to have a huge variety of interesting potatoes but not always as cheap.
I spend happy hours going through all the catalogues comparing prices and seed counts, then send off orders to several different companies. Because I order a lot of seeds I don't usually end up paying P&P, which you would with smaller orders.
Over the years I have tried different varieties of each vegetable and have come up with a list of my favourites which do well in my particular veg patch, and I tend to stick to those, but I try a couple of new things each year too. My resolution is to try seed saving but so far I have only done beans, which are easy.
Last year my brother tried some special offers from Lidl and not many germinated. Often seeds bought from the racks in shops have not been stored in ideal conditions so may have been spoiled, whereas seeds direct from the source should be safe. If you have packets with some seeds left in you can store them for next year if you reseal carefully and store them somewhere cool and dry - definitely not dumped in the potting shed or in a jacket pocket.
Happy hunting.