There are lots of websites offering and selling wide variety of a cell phones replacements parts and tools. One of the best website that I ever choose is Repair Universe, It is because they also provide a wide variety of tips, guides, recommendations and practical advice.
They provide replacement products, lcd screens, batteries and other spare parts for Zune, iPod, iPhone, Sony Ericsson, PSP, T-Mobile, Samsung and HTC & Nokia cell phones.
They offers you the information and advice you need now to select the right parts on iPhones, iPods,HTC, Zune,PSP,Blackberry,Sony Ericsson,LG, Nokia and a lot more. They also provide the proper tools on fitting and fixing your cellphones and gadgets for a very affordable prices.
One also I like in this site is that you can contact them not only via e-mail but also on a online chat features they have.
Here's one example video guide and instruction they provide for their costumers,
I almost forgot, they also provide same day shipping and delivery from the USA based costumers.