I know I have an old Grenadier knight with one. Also GW made some fanatics for their Mordheim line some years ago- a few crazy wildman type guys with huge flails.
edit: also there is a unit in Warhammer Fantasy (named: Empire Flagellant Warband) that uses Flails, though there are several Minis in a package and probably too expansive for just one Miniature...
I've got some historical minis equipped like that. Hussites, mostly, in everything from civilian clothes to three-quarter plate. And a few generic medieval knights equipped that way too. Old Glory made the Hussites, and I assume still do.
If you can use a pin vice {tiny drill] and superglue, you can give a flail to any mini you want.
Miniature's hand
Weapon Head
Drill Bit
Wire bent into loop
Necklace chain
Superglue or Epoxie
You can also make the weapon look as in mid swing by holding the mini so the chain hangs in the direction you want, then applying superglue to the chain links. The glue once cured will hold the chain in place barring abuse.
And if you want the flail to look like it was made of rope, take some wire and twist them together using clamps, vice, etc. How many wires you start with depends on the thickness you are seeking.
I'd recommend Reaper's Bones line. Find a mini with a flail and a second one you like. Cut the flail off and superglue to the second figure. Paint then done.
The Bonsium the minis are made of takes super glue really well and should hold tight, especially with some paint on. I glued a thin polearm to one mini and it's held up to some punishment.
You might try checking out some of the Asian-themed mini lines. I know I've seen monks with things like 3-section staves and samurai with tetsubo...something with a 2HD flail might be lurking out there.