IZ is an online retailer that offers clothing specifically designed for those on wheelchairs:

• Usually clothes are designed assuming that the wearer is standing. They can look awkward the wearer is sitting down. IZ’s collection assumes a sitting wearer.
• Blazers have an arch cut into them (as pictured above) so that pressing into the wheelchair doesn’t cause the back of the blazer to become wrinkled.
• Pants are altered to reduce the amount of bunching in the front and riding in the back.
• Shirts drape at the waist instead of lower down.
• Magnets, zips, or Velcro strips might take the place of buttons to make it easier to get out of the clothes.

Read more here.

Source: IZ

Via: Marginal Revolution, Kottke