Your Local Hobby Shop - This should always be the very first place you look, particularly if you're not super concerned about saving a buck.
So, to start with, you need to find a reputable place to buy.
Or, finally, you've caught the bug and become a dice goblin, and you're wanting to expand your collection. Or, you're a hobbyist - you're already way into tabletop, but you've just about rolled the corners off your first set, and you're looking to buy another. So, you're a beginner - you're getting into it, and you're tired of having to borrow the GM's crusty cheeto dice. A lot of my players come to me for dice advice - so I figured I'd write a guide for buying dice, whether you're a beginner, a hobbyist, or a collector. What started as me wanting new dice for every game I played ended in me buying a new set each week - at this point, I have over 100 matched sets, and over 1,000 loose dice to just play with.