Couple ways you can go; In the USA the BP7Es is not stocked, but it is in Europe (several Euro cars used it for a long time).
You can switch to non resistor plug caps and use BPR7es which are found everywhere.
There are other plugs that "cross" from the BP7ES, some swear there are couple that work fine. But beware; those "cross charts" are often little more than "it'll screw in the same hole"
5twins has link to a euro source for theBP7ES that's cheap and not too slow to get. I did that.
Or just torch your plugs dry, regap and keep going. I'm going to swear here that I've never used up a spark plug on an XS650. I think the plugs in Madness have 12K on them now and work like the day I put them in.
Only reason I buy plugs is cuz of the obnoxious asortment of will (don't) -fits I find in the barn bikes I buy.