For online electronic stores, try RS Components, Farnell/element14 and/or Mouser. As for cheap multimeters, better try Pasar Road.

Simple, get any USB to LPT/parallel/printer port adapter first (can find them easily). Then build your desired ADC to interface with that LPT/parallel/printer port. There are also ADCs that has integrated USB interface, though mostly were for audio frequency range... 

Of course you need to make the circuit board yourself, or simply use any prototyping boards. As for circuit diagrams, there are plenty of examples out there. Choice of circuit and components used very much depends on number of inputs/channels, type of input source (whether its audio or direct), speed (samples per second or sampling frequency range/scope) and sample resolution (number of bits). Anyway, what is the purpose of the ADC?

Earphone and "USB to audio"? Are you looking for ADC (analog to digital converter), or DAC (digital to analog converter)? A lot of difference there, the opposite of each other. If you hear some noise such as buzzing, hissing and/or crackling then possibly badcaps (at the output coupling and/or decoupling side), and/or noisy power supply to the integrated HD audio I.C. Nowadays USB soundcards are pretty cheap, and can find them in most places. There are also some affordable USB DACs cum HiFi soundcards...