In the USA? Kong makes a great one that I use on mine (Available at my PetSmart):

https://www.amazon.com/KONG-Ultra-Durable-Waste-Harness/dp/B0BBPZJSZ7

XL fit my girl but it's at the smallest size. She's a bit of a runt though. I'd say an average GSD would still be XL. They also make a "Tactical" one that's a little beefier with big side panels, but it seemed like too much for every day use. The one I linked once adjusted is incredible strong and incredibly easy to put on. Slip the hole over the head and buckles with (metal) buckles. Plastic doesn't cut it for when she lunges at a rabbit, from personal experience

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If not in the USA, when I was in Iceland I had one from Julius-K9 that was similar and also pretty nice. Just forgot it when we moved!