Racks are good to have, as they help support the load and spread the load at the same time. This helps keep the boat from getting deformed (not uncommon, as people sometimes strap the boat down too tightly). Good racks are not cheap - quite possible for you to spend more for the rack than you did for the boat.
Does your car have factory crossbars on it? If so, you likely can get a carrier that attaches to that directly, saving a lot of money. If not, you would at to go to a local sports shop and get a Yakima or Thule cross bars and uprights/feet. The uprights/feet are made to match your car’s roof. It may be possible to buy a used rack, but likely you will have to buy new uprights/feet to match your car.
Once you resolve crossbars, you then want to get the kayak adapter. Make sure the adapter matches your cross bars (e.g. thule cross bars and yakima adapters won’t always work together). There are 3 basic types: saddles, J-racks, and stackers.
- saddles keep your boat sitting flat, hull down. Can be from a simple/cheap foam blocks in the rough shape of your hull, to rollers/saddles, on to expensive options that help you get the boat on the roof. Easiest to load, often allowing you to slide your boat up from back. Requires wide bars if you want to put 2 boats on the roof, especially wide sit on tops.
- J-racks hold your boat at an angle, Large benefit is that with 2 sets of J-racks, you can fit 2 boats on a roof without having long cross bars. They have some similarity to the rack which help your kayak at the store, but spread the load much more to prevent the indentation you had (from https://forums.paddling.com/discussion/2938533/can-i-fix-this). Make sure you read instructions on how to strap - if I see a boat strapped incorrectly, it is usually on J-racks.
- stackers keep your kayak totally on side. Made to allow he most boats to fit on a roof, and most often used by white water boats. I wouldn’t recommend for you.