Is there anyway to find landing pages that are already pre-made with the right mix of the relevant key words, or allows you to plug in your keywords?

I wouldn't use pre-made landing pages. They really have to be unique towards your niche, product, or service..

Plus, you really think people would buy from your site? The landing page is what leads the visitor to purchase...if they see the same landing page over a thousand sites, that just makes them think its the same company trying to sell the same product/service across from thousands of sites...not good.
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While it can be good to learn from, EGS is right. If the user has seen that exact page before, then they may not convert for you. I guesse you could just hope the user has never seen that landing page. I would at least change them up as much as possible. Change the images, colors, wording best you can. When you start making some money, you can then have them made for around 30-50$