Right now the best way of getting a 1st party controller is to buy one used but in good condition. You should be able to get one in a good condition for max 35$ with shipping if you look on ebay, less if you find a good deal. Make sure that there are high resolution pictures available so you can judge the condition.
Maybe check your local craigslist or an equivalent website/app for local deals so you can see the controller up front.
I had a quick look and
this one looks ok.
Definitely upgrade to using a gamecube controller, it makes a huge difference! Remember that you will need an adapter, too. The
4-port mayflash adapter is the recommended one but they have a new
2-port version that is just as good (note that there was an old 2-port version that had issues). The "native support" of
this guide tells you how to set everything up, works for both the 2- and 4-port variant. I can also recommend you to start playing netplay over at smashladder.com . It will definitely help you as a player and show you what to focus on during training.