As a civilian, where can I get the gear needed and how do I practice ruck marches at home?

If you want to use what you'll be using at basic, you'll be spending some decent cash on a MOLLE ruck and frame...plus that ruck just sucks. Just get used to moving under a load. Any backpack should be fine. You are just starting so I'd recommend maybe no more than 35# (pound not hashtag) in your bag and don't forget a water source - camelback, canteen etc...
You want a decent set of boots/shoes, you will have issued boots at basic so remember that. You are just getting your shoulders, glutes, feet, legs, and back used to the movement. It is literally walking, with a backpack. Technique will come with time, just train the muscles.
Once you are in and allowed to buy your own equipment here's #1 and #2:
#1 - it's not boots, but SOCKS. Issued socks are a waste, get Fox River, Darn Tough, or Smartwool. They will make a ruck much easier on your feet and prevent blisters, provided...
#2 you have good boots. Issued boots are built to last and be as cheap as possible for the Army. Yes they do last a while, but they take some time to get broken in and they can be kinda heavy. There are lots of "aftermarket" boots out there - Rocky, Oakley, Nike, Danner, etc.. they are well worth the money. Coupled with good socks, an ALICE, Tasmanian Tiger, or Mystery Ranch ruck and the desire to keep going and you will be unstoppable and ready for Selection...hint hint.

You can purchase a medium rucksack and frame at an Army surplus store. Start light and build up to about 35 lbs or so. Same with distance, start out at a couple miles and build up to 12 max. Your final target speed / pace should be 4 miles per hour. You should be able to get into a ruck and frame for about $60.00.

A good set of well broken in boots are important so you don't screw yourself up prior to training.

Go to an Army/Navy surplus store and you'll probably find anything you need. As far as practicing, find a long road or trail through the countryside, then start walking. Start with a shorter distance, like a mile, and then stretch it out with each additional march. Stay hydrated, and make sure you have a cellphone on you. I'd recommend finding a partner too.