If you can’t get a doctor to prescribe an assistive device (ever heard a doc say they ~~wouldn’t want to condemn a young person to life in a chair~~), you should do research into what types of chairs will fit your needs. I’m guessing an ultralight would be most useful to you.
get accurate measurements with help from a friend. I did mine myself, but if you don’t have experience doing self measurement, like for sewing clothes, you might enjoy having a spotter.
Save searches on ebay for models and sizes that fit your needs, check second hand stores, charities.
check out wheelchair user forums, like on reddit or carecure [dot] net/forum/sci-community-forums/equipment
maybe check in on twitter’s disability community, people like crutches ‘n spice, !&&* on wheels.[br]
[br]
Best of luck. It can be so hard to get help from the medical system.