Are you sure you need a seat 22" high?
I've just checked my ma-in-law's old chair - it's one of those wooden armed, sit up and beg old lady's chairs - and it's only 17" high to the seat.
If it would be any good to you, you can have it - it's slightly cat scratched at the top of the back, but "works" fine. I also have a 4" thick wheelchair cushion, covered in fake sheepskin, that would make the seat higher.
The other alternative, if you already have a semi-suitable chair, is buy some elephant's feet/chair raisers - like these.
You can probably get them from that shop on the Wicker.
Or you could try asking the OT if s/he could arrange for the loan of some chair raisers? I don't know if they supply things like that if it's only temporary, but it's worth asking.