While there are different options to buy diamonds, it would be the best idea to buy a small loose diamond. When buying a diamond at any local store, it would be wise to ask for a 10x magnifying glass and use that to check if there are any impurities in the stone. You can buy loose diamonds via https://www.thebetterdiamond.com/.
If you wish to buy small and loose diamonds repeatedly for any purpose, it would be wiser to purchase a 10x magnifier for yourself. These special magnifiers are called loupe and are available online and from most jewelry houses.
The most advanced option to check a diamond is to use the stereoscopic binocular microscope which uses dark field lighting to enhance the view. However, those are costly and used in the labs or by professionals with an elaborate setting for diamond testing.
The most commonly available shapes for loose diamonds are baguette, heart-shaped loose diamonds, emerald, marquise, radiant, oval, round, princess, and brilliant.
Jewelers or goldsmiths generally purchase loose diamonds of different shapes and also sell those to the public. They purchase diamonds in bulk and use those stones in different jewelry they make. These stones are also embedded in watches, ladies' handbags, and even shoes.
While buying from a retailer who is asking for the lowest price is an attractive option, it might not always be the safest option. You should buy from retailers with a reputation and even then, you should not trust their words and ask for the certificate of the diamond. GIA certificate is the most trusted around the world for that.