I have ordered from this company:
Golf Cart Accessories, Parts and Golf Carts from Buggies Unlimited | BuggiesUnlimited.com
MadJax makes good quality GC parts.
Have never installed a rear seat kit, but usually just heavier rear springs are installed. They maybe included in the kits.
You can call Buggies Unlimited for tech support. They are very helpful.
There are probably YouTube instructional videos available.