If your top grate is the same diameter as a 22.5" kettle (21-1/2") you have some options.
1. Weber used to offer a SS grate...might have to call them
2. Weber "Gourmet" grate uses heavier rod.
3. Stok makes a cast iron grate ($35) but probably overkill for a smoker
Your lower grate is probably a smaller diameter if it's like the 18.5" WSM.
Maybe the charcoal grate from a 26" kettle would fit ?
Someone will be along shortly to verify grate sizes I am sure.
No problems here, even when loaded up with 4 pork butts to a grate, and I doubt my SS grate on my Performer is any heavier. If I'm not mistaken, all the stock 22.5" Weber grates are heavier than the ones from several years back that came on the OTS. That's the only 22.5" grates I'm aware of that I'd personally consider "flimsy", but who knows? You think that's the one that came on your wsm?
Weber sells a 22.5" stainless grate. Here is the link: http://store.weber.com/parts/detail/charcoal/enclosed-ash-catcher/1117
It doesn't specifically list the WSM as being compatible, so unless the 22.5" WSM top grate is different than the 22.5" OTG you should be good with this one, although honestly I would prefer not having hinges on a vertical smoker