It depends on what you are after, I have made cards for about 8yrs now and buy different things from different shops/ sources. I buy my card in and using a folding board, fold and cut my own to size. Envelopes I tend to stick to Craft Creations, although there are a few similar in cost/P&P charges.
Stamps, inks, embiellies etc I look around, often very similar in price so I'll use free P&P offers when they come up.
I must admit that whenever i get a nice card or a present thats been wrapped up using nice ribbon, i always take what i can from them for my card making box! Very tight of me I know, but its amazing what you can get.
I tend to make embellies from card and paper or use digi stamps - cheap and they can be printed over and over - 2Peas have a good selection inc. free ones or Jessica Sprague has digi versions of current and old paper collections by some of the big US manufacturers
Boyes have some lovely card making stuff I noticed the other day, stamps and things like that. Wilkinsons also have a few bits. Not sure of the quality. Some of the card making magazines also have some quite cool free bits with them but they can be pricey. Those book clearance type shops often have bits in for a pound also. (cant remember what the shops are called, publishers clearance or some such thing)
My dd bought 10 blank cards and envelopes along with some decoupage things in poundland and the quality was pretty good.