I have one but I'm in Switzerland. Postage costs would be high.
Around 10 bucks for the mail because of the package thickness.
By recollection, there were 2 types of Diana 5 caps. The older pre-1990 +/- were screwed on and had a screw to keep it from unscrewing. Seems like the later version was a plastic cap and I don't recall how it was held in place, maybe by a pin through the tube. If yours is the older type you might post here on the machining gate and see if someone can make a cap for you, of course you'd need to figure out what thread it has (and it's probably metric too). You might be able to get a new plastic cap through Umarex, at least it's worth sending them an email. Having owned both versions of the 5g, but some years ago, that's my recollection.
I think the 5G might have only had the plastic slip-on end cap. My pile of 5G parts is a 1978 model and it has one.@Whirligig - PM sent.