Check the list of Skins download links at the bottom of this page, where you'll not only find skins of the classic Superman look, but also the Kingdom Come Superman, Lex Luthor and more!
To use these skins, you'll first need to download the SimShow tool, which is used to import new skins into the game. Then, before you use the SimShow tool you'll have to manually extract the zipped skins into your Sims directory. It's a bit of a rigmarole, but here we go... Any BMP files should be copied into your /maxis/simshow/gamedata/textures/ directory. Any BMP files beginning with HFRO, HFLO, HMRO, or HMLO should go into your /maxis/thesims/gamedata/skins/ directory. Any BMP file with the word CAPE in it should be copied into both the above directories as well. Copy any CMX file beginning with B (eg B316MAFit_superman.cmx) and any SKN file with the word PELVIS in them, into both the /maxis/simshow/gamedata/cmx2/ and /maxis/thesims/gamedata/skins/ directories. Those CMX files beginning with C (eg C711Fa_01.cmx) and the SKN file with the word HEAD in it should go into both the /maxis/simshow/gamedata/cmx3/ and /maxis/thesims/gamedata/skins/ directories. There may be extra instructions for a specific skin, so make sure you read any TXT file within a skin's zip file. Once you've done all that, run the SimShow program and make sure you have the "Enable Filter" box unchecked. Then go through each of the four boxes at the top right - Bodies, Heads, Body Textures, and Head Textures - and piece together your Superman character from the file names in the lists. Quite often, it will only be necessary to select the files from the Body Textures and Head Textures, though you can have fun making fat, skinny, or even junior versions of the characters by using the different options in the Bodies list! ;) Then, you're ready to import the character into the game with a simple click of the "Import" button!