Suzuki made case guards for the GS500 in the model's early years, but the part was discontinued. They can occasionally be found on eBay and usually sell for $50 to $75, depending on condition and bidders. They are a two-point design that use the front two engine mounts to secure to.
A copy of these was made and sold by MC Enterprises. Part # No 1000-90. They are no longer listed on the website but maybe could be found on eBay.
Note: The MC Enterprises guards come with new bolts and instructions. The instructions say to use the longer bolt in the bottom motor mount, but the short one won't work in the upper. So, you have to use the long bolt in the upper mount. Then, the shorter bolt isn't long enough for the bottom mount, so you'll need to re-use the longer of the two stock bolts (the upper) for the lower mount.
These are by a German company called SW-Motech, imported through Twisted Throttle. The SW-Motech case guards are a three-point design, offering a better distribution of force than the two-point OEM units. The three-point design uses all three engine mounts (two front, one rear) to secure to. These will not fit F models.
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Don't forget to check other sites like eBay and Amazon and even google. You never know what a search for things like "GS500 crash guards" or "GS500 engine guards" etc will show ya!