I want to purchase a 240v elec space heater of about 5,000 watts. I have a 6-30R, in a 8x10 room. All the plug in heaters have a 6 ft cord. I need a longer cord from the heater at least 12 ft to put the heater out of the room into the 800 sq ft insulated garage. Do they make extension cords about 10 ft for my purpose. The outlet is about 10 ft from the breaker. Or can I make an extension cord with 10x2 with ground using underground cable.

Using underground cable as an extension cord is against the code because it is not rated as such. Go to an electrical supply store, get a piece of rated wire designed for an extension and install the proper ends.