Knowing what tractor you have is important. I bought a set from Titan. Nicely made but I sure am glad I didn't buy the higher weight model. I am glad I have them but they are way more limited in utility than I thought...
http://www.palletforks.com/skid-steer-attachments/pallet-fork-attachments-74/clamp-on-pallet-forks/clamp-on-pallet-forks-1-500-lbs-capacity-w-stabilizer-bar.html
First off, when you put clip-on bucket forks on your lower bucket lip, you extend the moment arm that is picking up the weight another 40 inches in front of the tractor. That really limits the capability of the tractor to lift the load. Plus, the pallet forks are heavy. Mine probably weigh 100 lbs all-up. That lowers my roughly 900 lb lift capacity to somewhere around 600 lbs.
The other thing is, pallet forks clipped on your bucket lip really put a boatload of strain on your lower bucket edge as the load bounces up and down. Several Orange brethren have permanently bent their buckets using forks. Not a good idea if you plan on lifting really heavy loads.
I just moved over 70,000 lbs of stone on my forks. I fortunately saw the risk associated with the flexing on the lower bucket edge. We modified how we carried the material, moving it back as far as possible and leaning it on the upper bucket edge. My bucket lip is as straight as ever. Here's what it looks like loaded...
I will soon be moving another 20 tons of rock into the back yard and will have to break the pallet loads down into lighter loads to protect my bucket.
Ray