I am looking to get started with creating my own kombucha. Is there any risk with getting a seed mother from a friend, or is it best to buy from an online store?
There is very low risk getting a mother from a friend and there are a lot of positives. I totally agree with @Laizer that they are quite resilient and it is obvious if a scoby is healthy or not. Black is bad, mold is bad, lack of effervescence is bad. It is not bad if it sinks in the liquid. I always keep a backup in starter tea in a small jar in my fridge just in case something happens to the ones I'm using.
The nice thing about getting it from a friend is you can tell how your tea will taste and choose accordingly. Kombucha, unlike something like Keifer, does not clone itself every time it grows. A mother is not genetically identical to the daughter it produces, and every mother is not identical to all other mothers. The organisms in your scoby (symbiotic culture of bacteria and yeast) affect how the fermented tea will taste, so you will always have variety. They also adapt, so if you are using a specific kind of tea for a long time, the daughters produced will start to adapt to the environment and perhaps be better tasting, or maybe not. If you really like the way your friend's tea tastes, you should get a daughter from them and if you like how the tea you make tastes, you should always keep the mother and give away the daughters or compost them or keep them as back up, whatever you choose. Also, websites/forums that you only have to pay for postage will allow you to get more than 1 or get another easily.
Some people, and I appreciate this isn't a mainstream thought, are of the opinion that you shouldn't ever buy a mother or something like Keifer grains because the organisms are so prolific that its offensive the the energy of the organism to get money for something it obviously wants to share. Definitely not a consideration for everyone, but if you are interested in energies that is one thing to consider.
If you have a friend who has a scoby, get it from them. There's really no need to get one from a store. Kombucha mothers and starters are fairly robust, and do a good job of fighting off fungus and other nasties that you're not interested in, and it's clear when they've failed to do so. It's hard to mistake a mold colony for a healthy scoby. In the sourdough world, it's a fine idea to combine starters with your friends, to keep them various and rigorous. I assume the same holds true for kombucha mothers.
Also remember that you don't need a SCOBY. You can just buy a bottle of organic, unflavored kombucha, pour it into a gallon jar, add some sweet tea and wait. A SCOBY will form without any more effort than that. So if you don't have a brewing friend and you do have a commercial kombucha that you like, just use that culture your own.