As far as I know there aren't any. Also I've never seen an AT power supply that doesn't have a built in power switch. Another problem is if there were an adapter you wouldn't have the 4 pin 12 volt connector required for all processors Pentium 4 and newer.
There are so many differences in the two specs that it isn't practical. Especially when using modern hardware. Besides the total wattage and the amperages on the various rails isn't going to be anywhere near enough.
Forget the idea. Older hardware and processors had radically different voltage requirements. I could go on and on but hopefully you get the idea. An AT PSU just can't fill in for an ATX.
Even older versions of the ATX standard can't run a modern system One reason was cited above. There are others.
When ATX motherboards first became available there were some motherboards that had both AT and ATX power connectors. However that didn't last long as ATX motherboards soon outstripped anything an AT power supply could provide. Not practical at all.
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