A/C grille extensions
There aren't special extensions made for this, but you can make a sheet metal sleeve, raw on one end with 3/4" flange out on other end. Make the sleeve long enough to lap into the existing boot by about 1"and then pop-rivet or screw inside the boot. Then screw the register to the flange.
You should be able to make one yourself easy enough by using a short piece of duct bought from one of the DIY stores like HD, but if you have a sheet metal shop nearby they should be able to make them for you fairly quickly, provided you supply accurate measurements.
"The physician can bury his mistakes, but the architect can only advise his clients to plant vines." -- Frank Lloyd Wright