I've seen all levels of quality grates. If you buy brand new grills, the OTS basic grill has the cheapest grate I have ever seen. I doubt it lasts more than one year at the most.
If you buy OTG's or better, the grates are somewhat better. If you go to buy replacements, the quality has decresased substantially. If you find the thicker grates at any store, you should grab yourself a couple spares. Home Depot and Lowes only seem to sell the flip grates. At least in the stores.
I'm thinnking that ACE hardware stores probably have some of the older grates around.
Happy Hunting!