Around here you can buy synthetic piss at the gas station for like $10 but I've never tried it. I used to use a clean friend's piss to pass probation drug tests while I was still smoking weed. He would piss in a cup or something and suck it up with a clean/empty visine bottle. I would then keep it tucked under my armpit until time to piss. Another interesting technique I have heard works is to drink a 32oz gatorade with something in it that you buy from the store... I forget what but I think the previous poster that said Bentonite might be it. Then you drink a second clean 32oz gatorade. Piss twice and the third piss will be clean and you have like a 2-3 hour window. This is a rough guideline from my stoner memory so you would need to check into it more, but this is probably the method I would try if I was going to use my real piss. You of course have your detox/flushing/masking drinks you can buy at a head shop, some gas stations, etc. I have heard you can contaminate a sample by having bleach under your finger nails and dipping your finger in the sample or pissing on your finger. Not sure how legit this is. Failing all the above, drink about 50 gallons of water so you piss straight water. Most places would make you drop again if too diluted, but might work for a pre-employment UA.