Some distributors don't want to carry our parts because they can't sell replacements for them. When you launch your boat, have it secured - preferably to your winch. It will roll off the trailer as you have never experienced it before. It winches back on easily too. Adding a couple of our keel rollers to a bunk trailer that has none, will greatly reduce the effort to launch your boat.
Adjustable brackets are widely available.
Just adjust them high enough so that some of the boat's weight is on the rollers. Every one of them is made in Ft. Lauderdale, Florida of the BEST polyurethane available for this application. Buy with confidence. Enjoy your boat and trailer.