Yes you can go on an online store and buy some markers, but what if you are under age, or do not have a credit/debit card? Well many things you use in every day life can be transformed to function like something you want. For example in a video on YouTube shows how a guy turns a plain glue stick into a marker.
If you for some reason can't make wheatpaste, or if it is too messy, then this is what you need. This goes for people in the U.S. only. Go to www.usps.com, find the 228 Label, they are free and you can order up to 500 at a time. When you have ordered the labels and have confirmed it by the phone call they send you it will arrive shortly. In the blank area in the center, you can write your tag and put it were ever you want.
At most hardware stores you must be 18 years old to purchase spray paint, but if you can't get the spray paint heres some help. Asking an older friend is easy at first but then it can get messy. At most large super markets they sell Visa Debit cards that are reloadable and only cost $9.95. You can buy one of these and buy as much spray paint or whatever you want, the only limit is your green ($$$).
Markers aren't hard to get! If you want just something to tag with go to a super market and get any large pen for example the sharpie magnum (not a gun), and the sharpie king size. These are just the basics, but if you want something more hardcore go to a local graffiti store (if possible) or do the same with the debit card described in the Spray Paint section.