Like roller was getting at theres really no one place thats best. If youre just buying 1 or 2 ungraded usually a local store will be best since the big sellers usually have stiff shipping costs for a single order. Usually graded ones
ebay is best if you keep an eye out, but on any given day any of them can be better.
ebay silvertowne provident metals
APMEX ect you really just have to check them all and see whose charging what and see if theyre any specials at the time. Silvertowne usually is pricey for raw ones though.