Hello @AndrewAtLiftOff !
There are multiple websites where you can buy barcodes. You can check the following out:
Hope this helps!
Can they be used for labels on clothing items?
And which of these services is more legit (as in certified) for commercial use?
I don't see why not; I use it for jewelry labels.
I don't think there's any "certification" needed for bar code labels. It's used internally to uniquely identify every item in inventory.
I don't see why one wouldn't use Shopify's free app rather than pay a third party.