Bed Bath and Beyond sells that same stool, just in white. Same price, except you can also use those 20% off coupons for it!
We have one. Will be taking it to WDW next month. The sinks are high! So are the toilets (higher than yours at home, I'll bet). Your only other option is to use the in room chair in front of the sink for washing hands, or hold up your child, which gets old really fast.
They will NOT provide stools. They don't have those on hand at WDW resorts.