You can find disease free bucks from reputable herds. Ask for referrals from people here, read up in the UCN and Dairy Goat Journal papers, etc. Insist on proof (negatiev test results) rather than taking their word for it. Such a buck won't be cheap, but he will be worth it if your herd is valuble to you.
A.I.:
Firstly, when I got into A.I. I didn't quite understand this, but a regular freezer isn't sufficient to store the semen. It has to be kept at -300 degrees or thereabouts. You will need a nitrogen tank, with liquid nitrogen to do this. I got my tank used for about $200, and it costs a little more than $100 a year to have it serviced regularly.
Also, you almost have to have a buck around to be successful at A.I., unless all your does are supremely good A.I. candidates and extremely easy to detect heat in, and you don't mind if they don't all get settled every year. I have yet to meet anyone who has a 100% success rate on A.I., particularly without a buck to assist in bringing them into a rip-roaring heat. It can be a little buck kid from the spring, or a pygmy that you'll never breed to, as long as he's smelly.
Places to buy Semen:
Biogenics: I won't do business with these people. I don't like them, and the product is very high priced.
Frozen Assets (Karen Lewis): This lady is extremely nice, the semen I've bought from her was viable, and the prices are good. Selection is good.
Buck Bank: I bought my A.I. kit from this lady, and it seemed like it took her forever to send it to me!! If you can, I recommend buying from her in person. She is nice, and the semen I bought from her, while it tended to be older bucks and not cutting edge stuff, was viable.
I don't think Magnum Semen Works is in business anymore.
A.1. is another company....I don't remember much about them except that they were the only peopel I could have bought Sodium Oaks Sasin from, and I didn't discover this sad fact until after I'd already blown ALL my money!

They had very good selection.
You don't have to buy from a company, many breeders have tanks and will sell just a few straws of what they have. You can ask on your district or breed list on the yahoo groups, or inquire from reputable, top name breeders in your area. What breed do you have, and in what area?
Lastly, unless you are already experienced in A.I. or are blessed to have soemone who is doing the actual breeding for you, buy inexpensive semen to start with. It doesn't have to be poor quality, just not the most recent flash-in-the-pan $100 a straw because his mom was first in her class at the Nationals this year stuff. Often the bucks who are proven and were cutting edge ten years ago but just aren't as exciting anymore for that reason are more affordable and reliable. My two cents.