Spectrum games are available for use in emulators in a number of file formats, the most popular ones being Z80, TAP and TZX, all of which +F can load. TAP files are at present the most reliable, although you should be able to use a lot of Z80 and TZX files as well.
One of the most authoritative sources for Spectrum games the World of Spectrum Archive, which has an extensive collection of TAP and TZX files.
Don’t forget that the classic Spectrum games we can play today on emulators remain the result of the skills and dedication of the programmers who developed them. While many have granted free distribution rights for their games, others have not, they are well within their rights to do so, and it’s up to us to respect them.
Therefore, please be considerate when downloading program files, and refer to the information here, here](http://www.worldofspectrum.org/permits/publishers.html) and here for full details on which publishers and developers have granted free distribution rights.