First check with the OEM to see if you can just buy them. Briggs & Stratton now owns the Murray Company and I would suggest contacting Briggs with your request.
To make them yourself, here's the decal paper, fixative spay, applicators and some how to information for Laser and ink jet printers: http://www.micromark.com/clear-decal-paper-for-laser-printers-copiers-and-micro-dry-printers-5-sheets,8001.html
Here's 6 pages of reproduction decals for lawn mowers: http://www.clickitandstickit.com/categories/Reproduction-Lawn-Mower-Decals/
Here's 8 different decals for the MTD mower: http://www.clickitandstickit.com/categories/Reproduction-Lawn-Mower-Decals/MTD/
Here's the kit for Murray: http://www.clickitandstickit.com/murray-11-decal-kit/
Here's the kit for the Murray 17.5 hp: http://www.clickitandstickit.com/murray-17-5hp-hood-decals/