@AshleyNicole06 Epsom salt can be found in pharmacies, grocery stores, etc. Usually it is with bath products or in the first aid aisle.
I wouldn't use regular salt because bumblefoot is a sore and sores burn when they come in contact with salt (unless really diluted). But, if you don't have Epsom salts warm water will be ok. The Epsom salt helps to soften the skin. I know some apply coconut oil and such to help too. If you see a "corn", you can pick at it and eventually a plug of chunky puss can come out.
Bumblefoot is actually a staph infection so it is important to take care of it promptly so that it doesn't spread.
Also, if you can take a pic of your duck's foot that would be even better so that we can see and not guess if your duck has it or not.