(11 years later than the question)
I don't know where it might be now, but my mother used to have it in our own garden - and made a point of how rare it was, being unable to find it anywhere commercially.
Sue Lee, to give her actual name, had basically educated herself into being an expert on roses. We had a Melanie Soupert in the garden of the house I was born in, in Chorleywood, Herts - along with many other roses (and other interesting things as well - a herb garden, several ponds and water features, a greenhouse for fruit, and so on. Never a dull moment.) And she knew of its rarity.
Later on, we moved to Wells, in Somerset. And Mum, being Mum, took cuttings of all the roses from our old home, and tried to grow them in the garden of our new home. Several of the ones which were supposed to be sturdier actually failed, but Melanie Soupert - surprisingly, perhaps, given that the yellow-blend type of roses of that era were notoriously fragile (a thing that Mum always used to blame on descent from the wild Rosa Foetida) - was one of the survivors.
Later on she planted over a thousand different roses, just one of each bush, in our garden which was between 1/3 and 1/2 an acre in size, and opened it to the public under the name of "The Time Trail of Roses". They were arranged in a time trail of date-order of the rose's breeding (if it was a man-made hybrid) or the date of its first importation to Britain (if it was a wild one from elsewhere in the world). We never really got a lot of visitors - our best days would have maybe a couple of dozen - but it became something of a tourist attraction for the connoisseur.
And as the visitors came around the garden, I would often be found, either helping at the gate, or doing my piano practice in an outbuilding that had started life as a garage and been converted to a studio-flatlet, in which I lived (while I still lived at home) or visited (when visiting after I moved away), with visitors often enjoying the music as much as the roses: I'm a professional musician myself, a pianist.
As for Melanie Soupert? Mum tried to interest some of the rose growers of the UK and even Ireland in trying to revive it, even going as far as arranging for some of them (I believe that Peter Beales was one) to take cuttings: but none of them survived, and our own Melanie continued to be the only one that we knew of.
Unfortunately the garden did not survive Mum's increasing ill health over years: and when she died in early 2013 - of long-standing complications from poorly controlled diabetes, complicated also by a couple of minor strokes and eventual kidney failure - the house had to be sold off, and the new owners appear to have just taken everything out and reverted the whole thing back to lawn and shrub.
Still, I'm glad to have since found out that another Melanie Soupert has made it back, even if it did come all the way from Japan...