Here's our technique. My wife dresses up sexy and we go out to a public place where black guys are likely to be. When she sees one she likes, she nods and I approach him. If he is unfriendly or unapproachable, we drop it right there and then. If he's a nice guy, I say something like "You got time? My wife would like to meet you. Let me buy you a beer." Usually he understands immediately or at least is curious. When he sees my wife, his eyes light up. We chat and within a few minutes he knows exactly what we want. My wife flirts with him and starts a bit of light touching to excite him, then discretely squeezes his crotch to check his size. If her eyes light up, we invite him to our hotel room. If not, we make some polite excuse, pay the bill, and leave. We have the advantage that I can speak with her in Vietnamese, so I can check she really wants to go for it without making the guy feel embarrassed.
We do this once or twice a month, and she ends up having sex with a hung black stud every few months, which is enough to keep her happy.