An adapter will be the best solution, but when it is not available in stores, you will lose your profit.
If your power supply is not under warranty, then you can buy any 8-pin adapter at your nearest store and then cut off your 6-pin connector on the power supply and solder the 8-pin connector.
The wires must be soldered and insulated with heat shrink
Chinese adapters are made of bamboo alloy and are very poorly soldered...
You need to check such points before buying. With a light load, nothing will happen, but while this adapter is working, you can order connectors, cable and tools in China, and make the adapter yourself.
I got a lot of these adapters from buying video cards. Previously, manufacturers put several pieces in a box with a video card and they are of very high quality.
Better to look for such adapters.
Anyone that buy new graphics card will have that adapter you are talking about cos they always come with the GPU package, OP probably don't buy new graphics card though, they are available everywhere online at very affordable price