Okay - horses for sale/loan/sharing etc can be found advertised in equestrian magazines, livery yards, online and tack shops. Sometimes also local newspapers.
What you need to look out for?
1) Is it healthy? Take someone knowledgeable with you when you visit, and make sure you get the vet out if there are ANY concerns. You also need to know how much responsibility you have for paying vets bills/farriery bills, etc.
2) Is it safe? Make sure you ride it. Take it on the road, in traffic, alone, in company, etc. Will it do something mental when faced with a cyclist?
3) Is it suitable for you? Is it young enough? Is it versatile enough? Can you use it for lessons? Is it fit?
4) Any special diet/ferriery? Kept out or in? Where will you keep it? Does it need to be kept at the owners yard? Can other people ride it with your permission?
5) Check out the insurance situation.
I have a headache, hopefully someone else can give you a more coherent list.