Can I purchase a longer 16gage wire for the lights?

You can purchase more wire for the unit at Lowes HOWEVER make sure you DO NOT exceed the maximum wattage of the transformer or you will fry it!

You can purchase addtional 16 gauge wire for the lights. You could splice addtional wire at the end of your existing cable or connect the old and new cable together with a landscape cable connector available in store. However, 75 to 100 feet is the maximum run of cable you want to use with 16 gauge cable. If you attempt to run 16 guage cable further than that no matter what size power pack you use not enough power will make it to the end of the cable to adequately light your fixtures. For longer runs you need wire with less resistance, either 14 or 12 gauge.