We sell tons of orthotics at the London foot pain clinic where we see tons of PF.
The main cause is over pronation and/or toe extension (toe running)
Custom made are a waste of time often they are poorly made in a back room and are really only meant for people who have server foot deformity (from birth, fracture or nasty accident). We evaluate the foot for £60 and sell a orthotic made to measurements and task for average of £60.00 inc VAT.
This is a fair costing for a very good orthotic, so look for similar prices.
Do you NEED orthotics to stabilise flat foot? That depends, research has shown that western shoes seem to cause a dropped arch (very rare in Africa where people often don't wear shoes). They won't make you lazy although I often give people a ankle balancing excercise called "pen penny" (worth looking up).
A pedi roller is a great device that allows you to regularly break up the fascia and slow down any thickening. I you don't have pain already this would be the preventative approach.
There are a page of excercise specific to self treating PF on the London foot pain clinic website
http://plantarfasciitistreatmentlond...tis-excercises
Good luck