They tend to be pretty rare and high priced. Your best bet might be posting in the merchanting folders of one of the unofficial forums inquiring after anyone selling one.

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Walsor Gryhm says, "Hmm, a most impressive weapon of note. I'll give you 16 silver coins for it."
You think to yourself, "This deal is getting worse all the time."

They're Auction-level items, so expect to pay in that neighborhood (be it coins, or trades, or services [Enchant & Ensorcel]).

I can put you on the trail of two of them, since Nuadjha & I recently traded, so I know that he has a 4x axe w/acid flaring one. I got the 5x discus w/ice flares (so faster, more likely to flare), and it hasn't really affected my hunting habits.
I am probably disinclined to trade, but I have no knowledge of his intentions.