This is always a tough challenge and can only be solved with compromises. In shoes like ballet flats, high heeled dress shoes, football boots and cycling shoes there is simply not much room in them. If foot orthotics are needed, then there is not going to be possible to get the orthotic into those shoes.
The first option is to convince them to change the shoes. Good luck trying that.
The compromise is to use a cut down style of foot orthotic so that it is not so bulky. The more you cut it down to get it into the shoe the less effective it is going to be and you eventually reach a point of no return in having a device that is so cut back it is useless.
The only other alternative that I have been using a bit lately is the instant arches type of product with is a self adhesive silicone product that gets adhered in the shoe. I tend to use them more posteriorly in the rear part of the arch area. They are not ideal, but they are a compromise that give you some effect and may get some therapeutic benefit.