antibiotic mixed w/agar on petri dish

is anyone familiar with this method of growing mushrooms? anyone know what the antibiotic is called and where you can buy it?

There are several , but you aren't buying yourself much if you want to do this with mushrooms. The problem with most antibiotics is that they dont' withstand the heat of sterilization. One way is to mix it in as the agar is cooling - THIS is a big pain. Now there is another substance that works very very well against all contamination and withstands heat quite nicely, but it disrupts the cells of mold as well - so it is usually only used for tissue culture AND it is quite expensive - a few dollars a mil if I recall.