Have you checked Craigslist, ebay, and your local and online tack shops for consignment sales? An older Stubben Rex in a child size is going to be difficult to find, and you may have to settle for something a little less rare.

 

Out of curiosity, why did she specifically recommend this hard-to-find saddle? I guess she has her reasons, but there are plenty of decent kids' saddles out there of all sorts of brands. Since she is 6 I would focus more on getting something the right size that fits her and the horse in question than a specific model. Stubben also has a number of other models that are available, and a Pessoa is always a good choice too. Just keep an open mind to other things available, provided that they fit and are of reasonable quality!

 

My kids are 8, almost 9. I tried for a couple of years to find a Stubben Rex for them. They aren't THAT hard to find, but because they're one of the few quality kid-sized saddles, and they're no longer made, they come with a steep price. You can almost always find them on eBay, but count on spending at least $250 for one - up to about $400. Supply and demand...

I had my kids first in a Wintec 15". They hated it. So did I. They felt clamped in. Then I had them in a 16" Collegiate pony-club A/P saddle with a deep seat and knee block and knee-rolls. It was a comfy, well-made saddle for both horse and rider, and they learned a lot in it. However, a couple months ago, I put my daughter in my 16.5" Passier from 1982 (flat, hard, and slick) and she was sooo happy. Even with it being too big for her, she rode in it better than in either of the other two.
My other daughter (same age - they're twins) rides in a Stubben Rex at lessons, and it does put her in an excellent position.
Sattlerei Kloster Beim Schonthal made a saddle that pops up in the ads now and then. It's called the Pony I, I think. Just as good as the Rex.

 

this is more than likely one of the Pony I saddles.

16" Old Courbette All Purpose English Saddle Great Decoration Christmas Gift | eBay

If you compare it to the Stubben Rex, you'll see they're quite similar.

 

Another thing to consider is that even though a 15" is probably a very nice size for a 6 year-old, it won't fit her for very long. You may want to consider getting a 16". It will still be a manageable size for her, but she'll probably be able to use it until she's about 10.
My kids no longer fit into a 15" by the time they were 7/8 (but they're amazons).

Here are some samples of nice hunt-seat saddles for kids:
Courbette 16" Close Contact Saddle | eBay
Stubben Saddle Rex | eBay
Childs Stubben English Saddle with Safety Stirrups Nice Quality Saddle | eBay
Thgis looks like a 16" - probably measured wrong
15" Collegiate English Saddle | eBay
Collegiate Close Contact Equestrian Saddle | eBay
Older Crosby Saddle | eBay
Crosby English Saddle | eBay
And then a splurge:
15 3 4" Nelson Pessoa English Pony Saddle | eBay

 

If you have a Facebook you can check the page called English Tack Trader. I'm always seeing Stubben saddles(as well as TONS of other brands) on there for sale. You can even post an "in search of" post and people can help point you in the right direction. =)
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I know this is an old thread, but I came across this on Ebay and thought I'd post it in case you were still looking

15 inch Stubben Rex Close Contact Saddle | eBay