Hey, @FourOne. Great question! It really doesn’t matter where you purchase your domain from. You should be able to point it to your Shopify store from any domain hosting platform.
However, have you checked to see if your domain is available to buy through Shopify yet? The advantage to purchasing it through your Shopify admin, is that Shopify Support will be able to help you with any questions or setup issues you may have with it. If you decide to purchase from a third party provider, we have a Help Doc for Connecting a third party domain, but you will need to contact the host directly for support should you need help with your setup, DNS settings or experience any issues with your domain.
All that being said, your domain can be transferred after 60 days from the time you purchase it; So, Shopify purchased domains can be transferred away from Shopify, or third party domains can be transferred to Shopify to host once the 60 days has passed.
Hope this helps with your decision. Let me know if you have any other questions or concerns.