Almost any 2-inch or 3-inch racing harness with shoulder belts will work with the G-Belt.

A 5-point harness, with or without the lap belts is suggested. See illustration.

We also suggest obtaining a harness that has springs on the shoulder belt adjustments. These help to prevent the shoulder straps from loosening as you drive. See illustration below:

2" belts will feel more intense than 3" belts. This is because the 3" belts spread the pressure applied by the G-Belt motors over a greater surface area. If you don't have a size preference, we suggest 2" belts, because you can always reduce the G-Belt intensity in software settings if you find it too strong.

Here are some 2"' belts/harnesses/restraints available at Summit Racing:

https://www.summitracing.com/parts/sim-29400bkr1x

https://www.summitracing.com/parts/rjs-1017401

https://www.summitracing.com/parts/imr-57346661

Here are some budget belts/harnesses/restraints available at Amazon:

https://www.amazon.com/BESTZHEYU-Compatible-Shoulder-Universal-Polyester/dp/B08HTZFLYH/

https://www.amazon.com/Aces-Racing-Harness-Padding-Certified/dp/B07TJ8R7FG/?th=1

I'd highly recommend this belt because the anchors can be easily removed making it very easy to run the shoulder harness through the G-Belt without damaging the belt. 

 https://www.summitracing.com/parts/cws-11152

I've recommended it to a number of other G-Belt users and they have all been very happy with it.