It looked slanted to me most placed I found it online, but I suppose another option, if you don't mind using XeLaTeX to compile, would be to use the unicode character ℒ directly. DejaVu Sans is just about the only font I have installed that has it:
I'll attach a cropped PDF with just that character too. I suppose if you really wanted to avoid XeLaTeX, you could include my little pdf with \includegraphics wherever you wanted to use it.
If you look carefully, you'll see that this is different from what I posted. In particular, it is more slanted.
I don't have a problem with \mathcal{L}. pechisbeque did. So I was trying to find an alternative. pechisbeque complained about one of the possibilities being too slanted, so I thought maybe the DejaVu symbol would be better.
But without knowing what pechisbeque disliked about \mathcal{L}, I can only guess. This thread is somewhat old though, and pechibeque never replied, so I assume (s)he found something (s)he liked well enough.
I realized that my default setup has eucal for the math stuff, hence our difference. I had forgotten; sorry bout that. The good news is that the original poster here doesn't have to look for a hard-to-find package. S/he already has it.
I'm posting a pdf of the eucal version of \mathcal{L}.