Most American lumber yards will give you access to varieties of Walnut, Cherry, Maple, Oak, Ash, Poplar, Mahogany, Cedar, various types of Pine, and perhaps a small selection of locally available woods.
If you want to get the "top quality" stuff, however, you are typically going to be making your selection from individually photographed and premium priced blanks one at a time, and buying them from an online vendor or at a specialty woodworking store in person.
Otherwise, you can make a selection based on a representative photo of the wood species that you are ordering and grab a few at a time, hoping that one or more may be what you are looking for .... in other words, a gamble.
I bought a 4-blank selection of Bethlehem Olivewood last year. 2 of them were a fairly non-descript straight grained style of blank, while one was of a diagonal cut. The 4th was a burl cut. Burls in olivewood are gorgeous, and tend to turn quite spectacularly (but not without some possible difficulties), and due to their relative rarity in woods, happen to command a much higher price. A diagonal cut is also a little rarer than straight grain cuts, as there is necessarily some additional waste created by cutting the wood at an angle rather than with the grain.
While the straight grained blanks created some lovely pens, which I sell at around 45 dollars, the diagonal grained pen sold much faster and at the higher price of 70 dollars, while I wisely kept the burl cut for a special recipient even though it could have commanded a price in excess of 100 dollars.
Thus, you can see that even within the exact same wood species, the end price that a customer would be willing to pay can vary wildly, even though the EXACT SAME pen style was turned (Comfort Grip, no rubber grip, replace clip with 24Kt gold plated cross) for all 4 pens.
But to answer your question ... the "best place to go" simply doesn't exist. All we can tell you is to look around, use your best judgement, and hope for the best! I gambled a bit later last year on a 1-pound bag of offcuts of the same Bethlehem Olivewood, and got 3 pieces of burl and consider it worth the price I paid just for that, even though 2 of those pieces weren't large enough to get a 2-barrel pen out of them. Even the 5 pieces of diagonal cut were worth what I paid for the whole pack, and I still have some of that pack still left un-worked...