Having trouble getting recommendation letter for masters
You are not required to provide a recommendation from people who are professors. What is needed is people who can comment on your ability to do research. While it is helpful to have recommendation letters from people who work as researchers (whether in industry, academia, or other non-university organizations), if they are not available, you have to work with the resources you have at hand.

What can such people comment on? Well, your ability to solve problems independently and creatively, as well as any capabilities or skills in teamwork and leadership would also be helpful. Other "soft" skills relevant to research—good oral and written communication skills, and so on—would also be helpful. Of course, just mentioning that you have the above abilities is insufficient; the letter writers should provide helpful examples to support their claim (just like in research!).

One thing to also note here is that having letters from your undergraduate professors is not entirely helpful if they only knew you in a classroom context. Three recommendation letters from such people would probably not be as helpful as someone who knows you in a more work-related environment.