Arguably the best way is to farm EXP off of the Alpha Chansey and Blissey that spawn (see the source for the exact map locations). They give a ton of EXP and will also drop an EXP candy.
Also if you're into glitches, there is a money glitch involving Ingo in Jubilife City. After using the glitch you can go buy as many EXP candies as your heart desires. I have attached a YouTube video that you can follow the process with. If you'd prefer me to write out the steps I can.
Source for Alpha Chansey and Blissey: https://www.polygon.com/pokemon-legends-arceus-guide-pla/22906449/farm-exp-candy-m-l-chansey-blissey-alpha-spawn-location/ Source for Money glitch: https://youtu.be/Dp5fwKwpIUc?si=PVSUsjALzNN-RU11