This sort of specialist equipment rarely comes up for sale pre-used. I did a quick search on the internet and only find two listings for this specific stander since 2014, both of which had sold quickly for a fraction of their cost new.
The advice on the Scope website https://www.scope.org.uk/support/disabled-people/equipment/used-equipment , lists three places that used disability equipment is commonly listed: Disability Gear, Ebay and Mobility Market . In addition to this I have also seen equipment listed in the forum of Special Kids in the UK http://www.specialkidsintheuk.org/forum/forum
You might also want to post a 'wanted advert' on any relevant Facebook groups or forums of relevant local organisations. However, I would advice extreme caution and ensure that you don't part with any money before you have seen the equipment in person.
Obviously, if you are buying used equipment you should ensure that it is thoroughly sterilised before your grandson is allowed to use it. I believe stream cleaning is the process that is favoured in many instances. The difficulty/expence in being able to ensure that used equipment is fully safe and clean is one of the reasons why many statutory agencies will not pass on pre-used equipment to families that might benefit from it.
If you can't find what you need pre-used and are forced to buy new you could try seeking charitable funding
https://www.scope.org.uk/support/disabled-people/search-grants or organising your own fundraising inititative
I hope your search is successful, Best Wishes
Jean Merrilees BSc MRCOT
You can read more of my posts at: https://community.scope.org.uk/categories/ask-an-occupational-therapist