you mean to convert from bigger jack to small one? any shop that carry electronic part will have it, I guess. I bought mine at sim lim, about $5 bucks (max), can't remember. Of course, if that what you mean
you have to get either a mono to mono, or mono to stereo. Look at the black bands at the steel part to idenitify what it is. Converters cost no more than 10 dollars.
ok... i presume you are going to record your guit into your com. check whether your amp is able to use 1/10" jacks (the kind you use for your earphones). if not you need to get a converter from 1/4" (the guitar cables) to 1/10" (earphone). and to differ stereo and mono, on the jacks, if you see 2 black bands on it, it is most probably stereo, and if it is just one, most probably mono.