Where can I buy beads like these?

Try a hobby shop.

Michaels and A.C. Moore are two companies with chain craft stores in various parts of the US. These and similar stores carry a wide range of findings and other jewelry supplies. You might not find the exact ones pictured but your creativity can produce a style all your own.

For the flat shapes in between the large beads, you could use a variety of different things such as colored Mylar cut into shapes of your choosing. And some of those could also be substituted with various size washers and other items from places like Home Depot and spray painted in appealing colors.

In addition to the stores mentioned, there are numerous online stores which carry an array of craft and jewelry making supplies.

I did a search for Beaded curtain kits and found dozens of sources. Here is one example.

I have a similar set. They were at Walmart where the teen bedding and decor section is. However, they are already strung. Well, they were until a manic 3 year old got them!