Power supplies are usually priced by power rating, and you're asking for 300W.
One solution may be a used HP 6651A for around $250. I doubt that you'd find a linear supply for much less than that, due to the weight of the transformers.
More modern supplies would be switching-topology, but are equally or more expensive (though much lighter and smaller).
Another option would be to gang up multiple supplies in parallel. Some of the HP 66xx series supplies support parallel operation.