It's available through suppliers of chocolate making supplies, and some baking and snack ingredient suppliers.
Google: "cocoa butter for chocolate making"
You can try IHerb.com and Nuts.com. Can't guarantee their quality or customer service.
Quality cocoa butter is expensive. And given the price of it, purchasing through a reputable supplier is important. Reputable suppliers pack to ensure the quality is not compromised in transit.
Santa Barbara Chocolate Company is a reputable online suppliers...but they aren't cheap.
If you have an artisan chocolatier in your town, contact them and ask if they will sell a small quality.
Yeah, I've always found, that if you need a small quantity of something you are making at home, but can't afford it or afford to buy a large amount of it, I find a small gourmet store or bakery that uses it and buy what I need from them.
I've never had any of them say "no", and thats because even though you aren't buying their baked goods, you ARE coming into their store/shop, which means you want to spend money.
If you do a lot of at home gourmet baking, find you a great private shop or bakery and talk to the owner/manager and tell them you would like to be able to get some gourmet supplies in small amounts sometimes. If they know you will be coming to them for supplies, they won't have a problem selling you what you want.
If they do have a problem with it, then find some other shop/bakery.