Try MasterTag. But there the only two that I know of.

I would love to find more solutions to tags also. But we find we're out of luck alot also.

We've wrestled with this for a while. This year, instead of trying to identify natives on the sign, or in a particular section, we will affix all signs to a thin wooden stake which will be spray painted green. We're hoping this will work. Natives can then be displayed with other plants and customers have a better chance of choosing the natives they want.

I work for John Henry, and we decide which tags to offer in our inventory based on customer requests, seed company introductions, etc.
My previous job was with a nursery, and one of my responsibilities was ordering the perennial tags-- from John Henry, Mastertag, and Merchants. I did this for 7 years, but never realized that new inventory tags would actually be added on my request-- I assumed that I'd have to order custom tags to get native plants or unusual varieties.
I guess that will always be true to some extent, for truly exotic plants, but as long as there is enough plant material in the trade to support keeping the tag in stock, JH will offer them.
There is a form in the back of the catalog, or you can submit requests via the website. It helps if you can estimate how many you might use (no commitment), and if more than one grower requests the same variety the odds are much better that a tag will be added. If you request now, the tags would be offered next summer.
Hope this is helpful....