I looked into factory wheel weights and thought they were pretty costly for just some inert lumps of cast iron.
Before you leap into weights, compare the weight with that of a box blade.  If you might be needing the blade anyways, it adds a lot weight further back than the wheels and with a lower C/G.  You will get more counterbalancing leverage with the weight hanging out the back than on the wheels.  It can also act as an anti-roll device of sorts if you start to tip, presuming you keep it fairly low.
I have a big block of concrete hung on the 3 point of my DX29 -- about 900 lb. -- which serves very well to keep things planted most of the time.  
I have weights on my wheels to increase roll resistance, which the rear weight doesn't do, but I made up my own from junkyard steel.  I've got about 130 lb. on each side with an investment of less than $20 in the weights themselves.