Where can I buy electric fencing for the best price?
What kind of electric fence do you want to install? 16 gauge wire, high-tensile wire, rope, tape? Do you want to go to a brick and mortar store or do you want to order something over the internet?
Is there a brand or type that works best?
It depends on what kind of electric fence you want to install. No two people might agree on the best kind or brand of electric fencing. I used to use 16 gauge wire, but I am switching over to rope because I believe that rope and tape are MUCH safer than wire for both horse and human, but, now that I have some of it up, I don't like the brand/kind of rope that I picked, so I will be replacing it and finishing with another brand/kind of rope soon.
What do I need when buying the fencing? I already have a fence in place so I don't need stakes for it, what do I need to attach the electric to my existing fence?
What kind of fence do you already have in place? Are you just wanting to do a "scare wire" inside of an existing fence to keep them off of/away from/from scratching or rubbing on either post and board, chain link fence, woven wire fence, or barbed wire fence? Or, did you take the wire and/or the boards off of the posts and want to reuse the posts for a kind of electric fence? You need insulators, but you have to buy T-post insulators for T-posts, wood post insulators for wood posts, chain link fence insulators that attach to the inside of chain link fence, etc. What kind of posts do you have? If you have wood posts at your corners and T-posts as your line posts you will need both wood post insulators and T-post insulators. Also, insulators made for wire or rope will usually accomodate both wire and rope, but the wider tapes (usually wider than 1/2") will require insulators made specifically for tape.
And can a buy a few stakes separately if I choose to divide a paddock in half?
Do you have a farm and/or feed store in your area of the country? Something like a Tractor Supply or a Southern States, etc.? You can often buy wood posts, T-posts, step-in posts, etc. at a farm or feed store. Also, if the person behind the counter is worth their salt they should be able to answer at least some of your questions and tell you which products you'll need for the kind of electric fence that you choose to install.
Are you friendly with any other horsepeople in your area that you could ask to mentor you on this project?