There's a fletcher in Solitude.
His inventory is going to be scaled to your level, but he sells bows and ammunition.
EDIT:
So I guess technically there is no "best" place to buy any item in the game. All shopkeepers seem to just get a random inventory every couple days when their shops reset and that inventory seems to be scaled to your level to some degree (As an example, my character didn't start seeing ebony ingots on the blacksmith vendors or daedra hearts on the apothecary vendors until I was at least like 20-25+)
With that in mind, traveling to the Solitude fletcher probably isn't going to net you any better gear aside from you potentially gaining some levels on the way, or the random number generator gods smiling on you.