Unfortunately over the years many Palomar 300a amplifiers have been separated from their power supplies. When the amplifier falls into someone's hands who's unfamiliar with it, they often don't know this is a two-piece unit unless they've seen it connected. In that case the power supply usually gets discarded later.
If you find the right version to fit your unit, you could expect to pay $100 or more for a used one. Since each of the six tubes also costs nearly as much, you'll want to have them tested before proceeding. If the tubes are not good, there is no point in wasting the money to retube it and buy the transformer.