Most people didn't buy them in the old days. Typically they were given away as promotional items. They are actually pretty useful beyond their obvious function of keeping your pockets from being destroyed by ink leaks and sharp points. For example, you could switch a set of writing instruments you use everyday from shirt to shirt without having to disassemble and reassemble it. But even the nerdiest these days realize that the only surer way to look like a total social misfit without involving body odors is to break your plastic-rimmed glasses at the bridge and repair them with tape.

Stationery supplies sell them. Baumgartner is one brand.

http://www.amazon.com/dp/B001ALPJIG/ref=asc_df_B001ALPJIG242...

I use to have an MIT one that I assume came from the COOP, but no idea if they still have them.

Edit: ooh, I just found a leather one on thinkgeek http://www.thinkgeek.com/product/f216/