Stock photography sites always have a way to filter search results by “vector”. And remember, no matter what format you get when you download something like that, you can generally open it in Illustrator and thus alter it and Save As SVG.
Speaking of Saving as SVG from Illustrator, you’ll always get this dialog after you choose the save location:
Those are the two most important settings I find. For the complete low-down, check out this comprehensive review of all those settings by Michaël Chaize.