If you know the manufacture of the part, and the part number, you can goto their website and get the spec sheet for that component.
The spec sheets always have a mechanical footprint. A word of caution: some footprint drawings have dimensions in mm and inches. One is the controlling dimension, the other is an approximation. Make sure you determine (from the notes) which is the 'real' dimension
if the pins on a 100 pin integrated circuit are spaced 1mm, but you use the approximated 0.039" spacing between pads, then your pads will be off by one at the corners of the IC.
A shortcut to finding datasheets is the digi-key website
www.digikey.com. They sell parts from thousand of vendors, you can search on the part number and usually they will have the datasheet / spec on their website.