Panties used during the shoot

I am curious to know, you use so many pairs of panties during your shoots. Do you supply the panties that the models pee in or do they sometimes use their own panties to pee in? I am sure you have a very big account at a female underwear store if you purchase all these panties yourself. Just curious to know how it works.
That would be interesting to know.....maybe a combination of Both. I've bought some girls panties before so they could pee in them for me.