I have this problem with my DD. She is very allergic to milk, though getting a little better. Up until recently if you had acup of tea with milk in it and gave her a kiss even 20 mins later, even a trace of milk on your mouth would leave an outline of your lips on her skin.
It meant that either I was stuck at home exclusively breastfeeding her for the last year, or getting a prescription for the only formula that would suit her dietary needs as it is seriously pricey.
The thing is, most people with milk allergies can't even have soya as a substitute because they will have the same reaction as they do to milk. I tried DD with soya yogurt to see if she could have it and after it built up in her system, she couldn't tolerate it either sadly.
Now because she doesn't get the bacteria most people get from dairy or dairy related products, she is lacking in good gut bacteria and has really bad flare ups of things like thrush.
I have bought her some freeze dried bacteria which is also expensive though less so, and add this to rice milk or juice for her.
Her diet is far more limited than those who only have a lactose intolerance and it can lead to further medical problems that require regular GP visits.
I am still breastfeeding to make sure she is getting as much from my milk as possible to compensate for whatever she may be missing out on, and I give her vitamins daily.