The cheapest STP I know of that is remotely shaped like a penis is the top of my head is the Mr. Fenis, which I've seen on Toolshedtoys. It's like $25. I've never used it, so I couldn't tell you much. You could also get the Mr. Limpy STP's, they'll run you about $35 unless you choose to buy just the packer and turn it into an STP yourself. There are several tutorials on how to do this on YouTube etc. The Mr. Limpy by itself is about $12, and to convert it to an STP you'd need silicone tubing and some sort of receptacle, usually a medicine spoon, and that would cost another $5 tops. So if you make it yourself you'd save quite a bit.
I'd suggest getting the xsmall Mr. Limpy to start out with to get used to packing, then either convert it to STP or buy something better when you can.
And yes, you'll use a harness. This is what I use, works great and was free. I made it with two shoestrings
http://www.ftmguide.org/images/simpleharness.pdf