Well HERE is where you will find most if not all the texture packs, and as for changing a texture in alpha it's fairly simple. I'm sure there's a guide somewhere. Which Operating System do you use? Windows, Mac, Linux?
For Windows XP, go to C:\Documents and Settings\USERNAME\Application Data\.minecraft\bin and open minecraft.jar with WinRAR or something similar.
Once opened, there should be a folder named terrain in there. Inside THAT is moon.png. I use Paint.NET but you can use Photoshop, GIMP, even just normal Paint. But MSPaint doesn't do transparency, so be careful.
When you're done editing, drag-and-drop moon.png back into that SAME folder you found it in. Easy.
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I'm in GMT+8. That's opposite to EST and plus an hour.