Chanel Boot Sizing, help!

Hi everyone!
Im a new member and specifically signed up hoping one of you Chanel ladies can help me locate a pair and confirm my size! Im wanting the basic black leather ones. In my chanel ballet flats and loafers Im a 39.5 (I have one size 40 but its because it was a back up black pair from sale and its all they had. The the stretchy kind so they fit fine) I have a pair of velvet knee high also in 39.5. With all those I don’t wear sock or slim stockings, what size would i want in the combat boot? 39.5 or 40? and with that said…..anyone know where i can buy a used pair or anything? I see them come up on eBay but always worry about authenticity and sizing. Help a sister out! Thank you in advance for any advice or suggestions on where to buy.