Well, I have something you might like.
My solution is one simple word: Lego. That most hallowed of modeller's shortcuts.
Specifically, I am referring to the web site 'BrickLink', and the particular part I am referring to is part number 40341 in the Bionicle catalogue. You can view it here:
http://www.bricklink.com/catalogItem.asp?P=40341
These are just about the right size to fit on a Dreadnought or Deffkopta perfectly, and require minimal conversion aside from hacking off the cross-shaped rod at their base with a knife. Closing the hole in the side is also probably best for aesthetic reasons (plug it with
GS or styrene, whichever you prefer). In either case, it's hard to beat how easy it is to whack these onto a model. Just make sure you come up with a decent-looking arm for it!
If you follow the link above, you will see:
Lots For Sale:
(Add to My Inventory | Sell in Auction)
Green (52)
Flat Silver (9)
Metallic Silver (2)
If you click on the particular color of the part you want, it should take you to a page where it shows all of the sellers, the quantity they have available, and the cost. You can get these things (at the time of writing) for five cents and the cost of shipping. (I bought about thirty from a single seller one time to make it worth my while to pay for $5ish shipping.)
The color part is largely irrelevant if you plan on painting your minis. Lego plastics take paint just the same as anything else, so no need to worry about that, though I might be wary about the metallic silver parts, since they're probably dipped in some extra substance.
Even in the days after Bitz ordering, you can still rely on lego.

The site's actually good for finding some other useful parts like engines or vent-looking bits, but I'll leave it up to all of you for that.
As a final suggestion, you can make an arm pretty easily with a bit of coat hanger and some solder. Take the coat hanger, bend it in a curve about 90 degrees, and snip a dreadnought-arm-lengthed size of it. Then take the solder, and coil it around the hangar. Smack the
CCW arm on the end and viola, dreadnought arm. The general idea comes from Victoria Lamb's rather cool diorama:
http://www.coolminiornot.com/37085
Go forth and krump!