Recommendation letters when changing between two unrelated fields
If someone in your undergrad math department has actually evaluated your work, then the year and a half is not an obstacle for a recommendation. Best if they know you as more than a body who occupied a seat, of course. 

But in general, if no one has seen your work, nor had a chance to evaluate it, then they can't really attest to it, ethically or practically. 

Since your situation is described as a bit unique, let me suggest, at least, a unique solution. You could ask a mathematics professor, either at your original institution or elsewhere, to evaluate your skills. There are at least two ways that could be accomplished. They could just give you a final exam from some recent course and decide whether you have the skills. But a fifteen minute or so oral exam should be enough to get an idea. They could then, simply certify about what they did and how you did. It wouldn't be a real letter of recommendation, perhaps, but might fulfill your needs. 

An alternative is just to suggest to an admissions committee that you would be willing to take such an exam if there is any doubt about your qualifications. That alone might be enough. 

Thirdly, nothing at all might be needed. The program seems pretty flexible. Perhaps flexible enough to accommodate your self study as valid.