235 hydraulic lifters are an oddball diameter and height. You're not likely to find an inexpensive source, and if you do, the worn-out camshaft that's causing the problem will still be there. Try adjusting the lifters according to the service manual instructions, and if that doesn't help, bite the bullet and replace the cam and lifters as a set. Putting new lifters on an old, tired cam is like throwing money down the toilet.
Before going to that extreme, remove the rocker arm assembly and inspect the underside of the rockers where they contact the valve stems. Quite often, there will be a step worn in the rocker face that makes noise even when the lifter is working right. Rocker tips can be refaced by a machine shop with the proper accessory tooling for a valve grinding machine.
Jerry
