If you are teaching in a non-traditional location, you need yoga mats! Particularly if you are working with people who may not have or be able to afford their own mats. How can you get mats? Of course you can just purchase mats, but finding donated mats can be a nice way to re-purpose unused mats and of course to stretch your budget further.
Reach out to likely donors. Yoga studios often have piles of lost-and-never-claimed yoga mats that you could take off their hands and put to use. If you teach yoga, students may enjoy donating their mats when they purchase new ones if their mats will be used in yoga service. Reaching out in person will be nice, but my experience is that happening upon the person responsible for donating mats may be unlikely. Email is not a bad starting point.
Explain the work you will do and approximately how many mats you'd like to collect total. This is a bit of a catch-22 because you probably need yoga mats before you can begin teaching. I think it's wise to at least begin setting up a class (being truthful that you have not yet acquired mats) so you can at least let studios know who the class is intended to benefit. In my experience, some studios have many, many mats they'd love to unload - since I could not use or clean or store 75 mats, I wouldn't want to show up to pick up an unspecified number of mats and be presented with that many.
Be mindful of your wording. I have mentioned that I'm looking for "gently used" mats. I wouldn't refuse to take mats that are falling apart or badly damaged in other ways, but I probably wouldn't use them in a yoga service class. Mats are important, but so is dignity - if you wouldn't want to practice on a mat, respect your students enough to recognize that they probably wouldn't either - even for a free class. If your class hasn't yet started, be truthful that it is in the works but that mats are an essential component of bringing it to fruition.
Offer to pick up the mats and be flexible with the time. Yoga studios often operate right before and after classes but are closed in between.
Express gratitude! Maybe write a thank you note. I've come across the idea of offering to send a picture of the mats in use, but be mindful of your students' privacy if you do this.
Once you pick up the mats, clean them - well - and sort out which mats are in fact in pleasant enough condition to be used.
Have a plan for how to transport the mats (very challenging without a car - is there a locked location on site that you will have regular access to?) and clean them on a regular basis after use (Lysol wipes are often a good and easy to transport option). Explain to students who might think to practice wearing shoes that removing shoes (and practicing in bare feet or if they really prefer, socks) helps prolong the lives of the mats.