Mai: Please provide reference pictures, it'll help us have a better idea of what you're asking about! :3
I assume you mean the yellow tape used by law enforcement to rope off areas, the kind that can be printed with "CAUTION" or "KEEP OUT." You can find this at most large scale home improvement stores, like Home Depot, Orchard Supply Hardware, or Lowes (I can't see your profile since part of CosCom is still down, so I don't know where you're from, but I will assume the USA, which those three stores are located in. You can probably find the same at the equivalent store wherever you live.)
If you plan on using it in a small amount, like the reference photo, it should be fine. You may run into a law enforcement issue if you try to rope off areas of the con with it or public areas at a park or something. Just like wearing police or military cosplays or carrying airsoft weapons, sometimes the public and/or real police don't know it's a cosplay prop and will treat it as a real emergency, which could get you in a lot of trouble.