Most of the grapple trucks I have seen are made by different Manufactures. Start with a chassis, add a grapple boom and then a dump bed. Do not know of anyone that actually manufacters the whole kit. Prentice model 120 with a 30 yard dump body on a International chassis expect to spend 80 - 100k new. There are some good used site around, google tree equipment and start searching.
Most likely you will have to put one together yourself essentially what you want to build is a chunk truck with a box on it. You should build it out of a tandem axle so you will have some payload left after you bolt on all the parts. What would be handy is if you had a roll off hoist on the truck so you have a couple bins when one gets full roll it off and pull on a empty box. If you got some heavier stuff that the boom won't lift put the box on the ground drag the stuff into the box and pull the box back on the truck.
There is plenty of used iron out there. Treetrader.com is a good place to start. I'd also check out places that deal in logging equipment such as skidders,etc. They usually have a decent selection of used grapple trucks as well.
