Depending on your state, you may be able to secure a brush truck from your forestry or land management agency, whoever is responsible for wildland fire control. My department has 2 military surplus 2.5 ton trucks that were converted by Georgia Dept. of Forestry for wildland firefighting, and they work quite well.
Since I just helped build a Brush Truck last year for another department and Work for a couple different contract crews that build our own brush trucks.
Nick made a good point talk to your local forestry agencies we picked up a bed from a FS type 6 for nothing they even delivered it for us. as for the truck itself we actually had the local power company that had a pick up they were getting rid of that donated it to the department. I know that same department has a contract with the local department of lands and were givin 2 tenders and a brush truck on a "Lease" of like 1 cent a year until they decide that they dont want them anymore.
Also another way to do it is look for local contractors or even local utilities companys that are surplusing or even giving away their old vehicals some are a little worse for wear but with a good mechanic can get it in good running shape pretty cheap then its back to the surplus from forestry agencies to equip the truck