Common Spell Components with gp value
Indirectly, any valuable spell component can help your wizard and druid cast spells, because it can be re-sold or traded for components they can use.
Components in common demand would need to be affordable by lower level, less affluent casters. Let's say anything up to 100 gp. Common components listed here are for spells from the Players Handbook, as many of the spells from the various extension books appear more unusual and exotic, and their availablity in the game is optional. (You can ignore the pearl and diamond if these have been taken).
- charcoal, incense, herbs (10 gp) - find familiar
- jade dust (10gp) - magic mouth
- lead-based ink (10gp) - illusory script
- gold dust (25 gp) - arcane lock
- marked sticks/bones (25gp) - augury
- holy water (25gp/100gp) - protection from evil and good/magic circle; hurts undead
- powdered silver and iron (25gp/100gp) - protection from evil and good/magic circle
- diamond dust (25gp/100gp) - nondetection, stoneskin
- ruby dust (50gp) - continual flame
- diamond (50 gp) - chromatic orb
- platinum rings (50gp) - warding bond
- glass eye, jeweled horn (100gp) - clairvoyance/clairaudience
- pearl (100gp) - identify
In addition, a substance that would be of high value for a wizard and should be available in any magic shop catering to them is the magical ink that wizards need for penning their spells, worth 50 gp per spell level.
Aside from these, you also could have spell books (50 gp), other books (25 gp) on arcane subjects, component pouches (25 gp), and spell foci (1 to 20 gp) on the shelves. You could consider useful alchemical substances, like acid (25 gp) or alchemists fire (50 gp), and tools of the trade like parchement(1 sp), paper (2sp), scroll cases (1gp), ink (10 gp), vials (1gp), silk rope (10 gp, for rope trick), and robes (1gp).
Lastly, although this is more costly and would be less common, you could consider a crystal ball (1,000 gp) as the high end item on display, because it is such an archetypical trapping for wizards.