I thought CPL polarizing filters were for vibrant skies?
CPL filters are the tool that is generally understood to make skies more vibrant. Though, because GND filters are able to selectively hold back light, they can produce a balanced photograph where vibrant color and detail in the sky is not lost in the process of properly exposing the foreground.
Technically, they
don't make the skies more vibrant... but by optimizing exposure they may allow you to better preserve the vibrance & detail that is already there.
Are there similar ones of same quality (I want a square block like those so I can choose the amount as opposed to a lens filter) for a cheaper price? If not, what one should I get? Since I haven't used one before I'm not sure how many stops down I want, what would be a common good one to use?
I've always used Cokin GNDs, so I can't really say how they compare to Lee filters in terms of quality. Nonetheless, they've worked out rather well and helped me tackle shots that I couldn't have otherwise.
I carry three of them around with me... an ND2, ND4, and ND8. I've found that I use the darker ND8 more than any of the others. In fact, in certain situations, I've even stacked the ND8 / ND4 or the ND8 / ND2. I don't recommend that technique (it doesn't work too well), though if you can find a darker filter than the ND8, I'd recommend buying that, as well. The ND2, while not entirely unused, is so light that its effect can be matched merely by a polarizer in many situations.
I think Cokins cost somewhere between $20 and $25 each, just for reference. Not sure how much the Lee filters go for. If I recall correctly, three Cokin GNDs and the lens-mounted holder cost me something like $75.