For a generic (low powered) foci, I'd likely go with the "General" spells, as those seemed to be used for anything at all.
If you really wanted to limit the exchange of foci, you could tie them to the person. . .game a while back, the GM used this rule for not only foci, but for things like PP multipliers. (Something along the lines of a week of ritual and fasting to tie the item to the user.)
For foci that offer major benefit, I usually made them hard to get, almost impossible to buy.
i.e. if a Dragon tooth athane was a good foci, an athane made from the tooth of the mightiest dragon of yore than offered signifigant bonus to rituals, you'd need to get your hands on in some way other than buying or making it. (it's either a signifigant treasure item, or something the party finds out about and quests for to obtain.)