Where can I buy 2X10X8 and 2X4 lumber that is not pressure treated for building a raised garden?

Both Lowes and Home Depot carry them. Usually HD is cheaper.

Home depot and across highway is Lowes. The wood is better at Lowes

Home depot , Lowe's both usually carry untreated wood

Yes at most Home Depot you should call and see if the store your shopping has it in stock.

Home depot sales treated and untreated lumber, just call your local home depot and speak with a lumber associate.