You could use an image or sigil you’ve grabbed online but it’s not as effective as creating your own seal.
When you use a sigil created by someone else you’re using a random symbol of someone else’s creation to focus your magical intention. This can still work and be effective but it is not tapping into your mind and your magic in the way a sigil is supposed to work.
The many ways that you can create and use a sigil is up to you the magician and your experience and experimentation to focus on your goals and ritual.