There already are some in Krita!
Go to Settings → Dockers → Symbol Libraries. You may need to change the drop down to ’ Pepper & Carrot’ speech bubbles. Drag these in as vector layers so you can scale them infinitely to any size, and then right click on the layer and go to ‘layer style’ for any colour borders/shadows/effects that you might want.
There is an expanded pack of vector speech bubbles to add on by somebody else ready for download in this topic here