You have a few options:
From a friend — honestly the best way. A healthy, well-loved SCOBY from someone who brews regularly comes with good starter liquid and a bit of passed-on wisdom.
Buy one online — plenty of reliable sellers on Etsy and specialist health shops. Just make sure it comes with plenty of starter liquid (Minimum 200ml)
Grow your own — you can actually grow a SCOBY from a good quality store-bought kombucha. Just make sure it’s unfiltered and unflavoured. It takes a couple of weeks and a little patience but it works and it's a great way to start if you're not ready to commit to a full purchase yet. Find my recipe here.