There are guidelines for purchasing tools, items, and the like. But I can't seem to find any for buying those of Superior or Excellent Quality.
The prices are at the end of C&G, in particular on p.144. But comparing the table of multipliers there with the tables on p.141f, you find space for SG adjustment.
Also keep in mind the rules for bargaining on p.74ff.
The availability of superior and excellent quality goods appears to be largely SG prerogative.
Cheers
I would just use the modifiers called out above for Superior items, but for Excellent items, I'd take some care. Bear in mind that items of that caliber are generally pretty unique, to the point where I'd say it should be very hard to come across one at all, much less the exact kind you need, if one of the PCs isn't themselves playing an extremely skilled craftsman. Such an item demands either a man of genuinely awe-inspiring capability or a very rich crafting guild in a likewise well-off city.
Given that, it might be worth treating as a plot occurrence. The seller won't necessarily sell it for a standardized price if it's a highly delectable piece which also represents the culmination of all their life's training and a good roll on Experimentation. (It has a different name for craftsmen I think, but it's the same idea, so I'm gonna invoke Noble's Parma here.)
In other words, they'll part with it for however much you think they personally value it, especially since competition probably isn't a concern in regards to a unique item. To spice things up a little, you could make it into a story; in a lower-wealth campaign where just one of these items is a really big deal, they might part with it after an adventure taken in their aid plus a token payment, while a higher-wealth campaign near a wealthy city might spend a story setting up a deal or long-term trade relationship with the guild.