I do not know a source of Excel formulas. Most people use the free Internet charting services or buy their own charting software these days. It is easy enough to get Excel data into most charting software. Trying to do it all on Excel is a bit like re-inventing the wheel!
The only suggestion I have is the book Technical Analysis From A to Z by Steven Achelis. He includes the formula and instructions for calculating many of the indicators in MetaStock.
Another book with formulas for many indicators is The Encyclopedia of Technical Market Indicators by Robert Colby and Thomas Myers.