There is a “wheelchair” that my Husband used called a Broda Chair. The easiest way to explain how it looked it was kinda like a Wingback chair. There were adjustable parts near the upper torso and shoulder that would provide support. The chair also tilted back so he would not slump to one side or the other. There are regular wheelchairs that have a tilt back that can help prevent sliding to one side. There are also non slip material that can be placed on a seat that can help.
My go to recommendation for people who sometimes need a wheelchair has always been to invest in a good tilt in place wheelchair with lateral supports and a ROHO seat cushion. If that isn't a possibility then ask for an OT assessment, they can help you find the best lateral trunk supports for your type of seating and budget.
There are regular wheelchairs that have a tilt back that can help prevent sliding to one side.
There are also non slip material that can be placed on a seat that can help.