I have had a difficult time finding inexpensive plant labels in mainstream stores. Here are some plant markers that are available online and are eligible for free shipping. You can write on them with a pencil, and then erase and reuse them.
Luster Leaf Rapiclip 6-Inch Garden Plant Labels - 50 Pack 840 ($4.74 + Eligible for Amazon Free Shipping)
Jump Start Reusable Plant Markers 50 Ct. ($4.95 + Free Shipping)