We always just play with the assumption that my wizard, who has dedicated years of his life to the study of magic, has the sense required to acquire whatever common reagents he needs along the way without my assistance. For something that requires exotic materials like a diamond, or 500 GP, then I will need to role play their acquisition.

And also whenever possible sleep in the attic of the nearest building, to collect all that sweet(?) sweet Guano on your coat...