Here's a tutorial that will show you how to make a standalone uploader that will program your Unos with a .hex file from an SD card:
https://www.gammon.com.au/forum/?id=11638
One of our forum moderators makes and sells pre-made Standalone Programmers based on the same code and design here:
http://www.crossroadsfencing.com/BobuinoRev17/Programmer.html