The best place I have found is in the UK. Wiggy Sports. I have ordered from other companies and Wiggy is the best. All of the other shops I have used simply send you loose letters for the name and they are quite difficult to correctly place on the shirt. Wiggy sends letters to you affixed to clear plastic that you place on the shirt. Makes putting names on EASY. I have ordered tons of numbers, sleeve patches and letters from them.
Their website is:
www.wiggysports.co.uk
Postage can be a bit expensive because the items are coming from the UK, but I feel it's worth it to have the letters affixed to the plastic instead of loose. They also ship VERY quickly! I live in the US and generally get items from Wiggy in a week or less. If they don't list the specific player you need, simply email them and they will put it together for you at no additional charge. If they don't have the Away numbers and letters listed, email them and they will get them. I can't say enough about these guys. They are a great store.