How is it unfair to be charged extra for an alternative if you can't have the standard item? I pay more for lots of things because I'm a coeliac; if it costs more to provide, it costs more to provide.
There's a political/environmental argument to be made for encouraging coffee shops to subsidise their fake milks, sure, but outside that, it's normal to have to pay more to a private company if it costs them more to cater for you. Sometimes you can persuade a company it's good PR to provide the more expensive product you need for the same cost, but they're not obliged to.
The place to argue for help with the extra costs you face due to medical conditions is government — either government subsidies for private businesses to help them equalise costs for people with medical conditions, or direct financial help for people with them.