I doubt you'll find anything off-the shelf as it is not a standard value. Why don't you buy a 24V version and remove some turns to bring it down to 20V? Secondary is always wound on top and it should not be a problem.
No minimum qty. 17€ is the price for a one off. Just add shipping. And 3 weeks delay...
They ship anywhere. shipping may cost as much as your transformer tho...