Hi Gess,
I had the same concerns about getting one for my DS, since he is not only mobile but quite a sprinter.
After trying many places (even freecycle) and much hitting of brick walls, I was finally adviced by my old portage worker to approach the school physio to cut down on red tape and paper work, I was also warned it was a long shot but worth a go.
So I phoned the school physio, explained why I needed one, the fact DS is an escaper, has no sense of danger and that he also refuses to walk with no warning.
We agreed to meet each other at the school, completed the forms together and crossed our fingers.
Within a month I had a call to say they were delivering a major.
Turns out they actually loan you the buggy and it remains the property of wheelchair services, but that is actually an advantage as they take care of all maintainence and any repairs needed.
You can get your OT to refer you, but going through the school physio cuts out the need for a visit or assessment.
Not sure what age they go up to, but they are pretty big, my DS looks tiny in his.
There was another boy at the donkeys on Saturday in one and he must of been 10 or 11.
Hope this helps a little, only trouble with this method is unfortunately it is the school holidays now.
Good luck with whatever you try anyways.