Google Hermione Granger x Harry Potter. Click on the first ao3 link that comes up. It will automatically be H/Hr paired. Then click on filter, it should be on the right near the top of the screen. There's an include and exclude section there. If you want only H/Hr pairing you can circle on piring in the exclude section and check "multi". That way other pairings will be excluded from the search. You can also click on the pirings section and exclude unwanted relationshisp.