I did this myself on my last cart. There are a number of kits sold online - I got one with pretty good seats, and it cost me $700 for the kit. I also purchased the upgraded springs for a heavier load - it was about $50 for those parts. Took me an afternoon to install it.
My kit was by MadJax and I got it from TNT cart parts -
Madjax Rear Flip Seats - Fits Club Car, EZ-GO, Yamaha Golf Carts
It's not that hard.