Your chances of finding a white couple suitable for both you and your husband's sexual desires would be increased, most likely, if you were to visit a swinger's club. If you live in or near a (somewhat) large city, the chances of finding one relatively near-by will be better than if you live in a small town, in which case you might have to do some driving.
As I understand it, most swinger's clubs accept prospective new members on an "observation only" bases; meaning, they cam come and observe the action, at least once, without being expected to participate, unless they want to.
If you visit such a club, where... by definition... married couples want to have sex with people other than each other... but you get a bad feeling about it for any reason, I would suggest leaving and looking for an alternative swinger's club.
There may be such clubs that seek members attracted to biracial sex. If so, such clubs would maximize your chances, it would seem, of finding what you're seeking. This would be something you could enquire about when you first contact a prospective club.