But how close they are to standard... and how big...
No idea.
It might be best if you can find someone local that raises geese... so you can see exactly what you are getting before you buy.
Facebook has clamped down on animal sales, but it is still a good site to find local animals for sale... they are just worded differently, and no longer in a "for sale" post.
Craigslist can also be a good place to look.
Also, the bulletin board at your local feed store... and just ask them, they might know.
Holderreads had fantastic stock...but I heard that they have retired... I have no idea who took over their stock.