When CTAN was young, most people would start using TeX with a 300 dots-per-inch (dpi) laser printer, and sets of Computer Modern bitmap fonts for this resolution are available on CTAN. (There are separate sets for write-black and write-white printers, as well as sets at 120 dpi and 240 dpi.)
Over the years, there have been regular requests that CTAN should hold a wider range of resolutions, but they were resisted for two reasons:
If your output is to a PostScript-capable device, you advised to switch to using Type 1 versions of the CM fonts. Two free sets are currently available; the older (bakoma) is somewhat less well produced than the bluesky fonts, which were originally professionally produced and sold, but were then donated to the public domain by their originators Y&Y and Bluesky Research, in association with the AMS. Unfortunately, the coverage of the sets is slightly different, but you are advised to use the bluesky set except when bakoma is for some reason absolutely unavoidable. In recent years, several other 'Metafont' fonts have been converted to Type 1 format; it's common never to need to generate bitmap fonts for any purpose other than previewing (see "previewing documents with Type 1 fonts").
This question on the Web: http://www.tex.ac.uk/cgi-bin/texfaq2html?label=getbitmap