Just type Print on Demand into your search engine and you will find many more
Different sites tend to do better for one type of photography than another would, for example Photo4Me does very well for Landscapes whereas Red Bubble is good for more graphic type designs. Take a look at these different sites and see whats been selling lately. You can of course upload to more than one. Some are free and others charge a small fee. Pay attention to how much you are selling your images for and the cut that these sites will take. I think its a mistake to sell cheaply. I always think that if someone is going to be buying a piece of bespoke art then they don't want to pay Ikea prices!
Make sure you have suitable titles, keywords and descriptions on your images.
It may take a while before you make your first sale but stick at it and take the time to learn all you can from some of the other artists that are selling well.
Set up your own page on Facebook and use your Twitter account to promote your latest work of art.
I've never had much luck with selling stock images. Probably because I find the genre boring! Taking photos of a garden spade or a tea cup and saucer in super crisp clean lighting is not for me but many others I know do very well with stock photos. They may only sell for a few pence in some cases but if you manage to get a great image it could sell over and over many times.
Some of the Stock sites have very strict guidelines for submitting your first images but once you are accepted thats it you are in.
I recommend getting the free report offered below. It has some very good tips about stock photography.
News Worthy Photos
Street Photography as its commonly called could possibly earn you 100's or 1,000s of pounds if you have captured that exclusive image! Much more likely though is that you have captured a news worthy image that can be used in local or national news channels and publications.