Craigslist or the junkyard is a source. Even Ebay.
And which type would depend on what kind of Jeep you have. If it's an early Wrangler or CJ you have the "sport bar" which will only allow lap straps. If you have a '92 or newer Wrangler you have a "family bar" and you can use shoulder harness belts (recommended).
I have modified my family bar to accommodate five point harnesses for my kids as well as headrests to protect against whiplash.
Need more detail.
It's kinda' hard to see it in this pic but I put a bar across the back at the top of the seat for the harnesses and the angled bars come up to hold the headrests and re-enforce the side bars (similar to a Rhino side by side).
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