Q: Where can I get a keyboard and mouse for a Mac SE?

Welcome!

"Where to get" inquiries are more easily answered if we know your approximate geographic location. No sense on our sending you to a vendor who does not ship to your location.

That said, one resource that often works for getting older hardware---regardless of where you live---are Mac User Groups ("MUGs"). This locating page can help you find if there is one near you:

http://appleusergroupresources.com/find-a-group/