https://www.svgshapes.in
https://make8bitart.com/
https://www.svgrepo.com/
That's all I know.
Oh also icons8.

unsure if I can link this but
piskelapp for pixel art probably (accounts dont exist there anymore btw)
As for vector art, there's a button to switch to vector in the costume editor in the bottom left of the costume-grid:

Note that it might not fix the quality of the image either way.
EDIT: changed the hyperlink to the create sprite url

If I remember correctly, you can shrink a costume down, click off then resize it if you want more pixelation.

Aseprite or Piskel are great for pixel art, and Inkscape or Adobe Illustrator for vector art. (These are just recommendations, you could choose other platforms)