If you want to buy kosher tzitzit strings online, beware! Many of the top Google results are non-Jewish tzitzit dealers who sell inexpensive tzitzit, but they are not kosher by any stretch of the imagination.
If you do not intend to tie the tzitzit yourself but to buy a tallit katan with tzitzit already tied, you will also want to be sure the tying was done by someone reliable.
The garment must also fit certain halachic requirements.
Answer:If you but tzitzit in a Hebrew gift-shop or Hebrew bookstore, you can check to see if it has a label of kosher supervision.