It's unlikely that you'll find a one-stop shop for all the terrarium supplies you need. More likely, you'll have to visit a number of suppliers such as pet shops, antique stores and garden nurseries to get all you need.
There are very few supplies that you'll need to run your terrarium because they're so self-sufficient and all you really need to do is clean and water them. Most of what you'll be looking for will be supplies to set up your terrarium in the first place.
There are thousands of options for terrarium containers. And even more to be discovered by you! Some of the more obvious places you can get glass containers that would be suitable for a terrarium are:
Decorations can be bought (or found and collected) from anywhere. And often it's these items that are picked up, showing a bit of wear and tear that really make your terrarium interesting.
Craft and antique shops are a fantastic sorts of bits and bobs, and the Internet can be a great source, just because the selection available is so huge!
Your best bet for plants is your local nursery or garden centre. If the selection is too limited, you can always turn to the Internet.
With terrariums where one of the main aims is aesthetics, it's a good idea to buy plants rather than seeds, so you can lay out your terrarium to your liking with real plants that you can see and move around if they don't look good in their place. You can also enjoy them from the word go rather than having to wait for seeds to germinate and the plant to reach a suitable size.