[William]

Here are a few simple systems, depending on your desires.

1.) Allow casting without material, but give skill bonuses for involving appropriate materials: +1 for wearing a bit of fur of the animal to be shapeshifted into (nonconsumed), +2 for drinking a vial of its blood (consumed), +3 for a well-made hood in the animal's form (an investment of much more time to create), etc. Assume that any mage can do this for various spells, but what materials are appropriate are secret knowledge. This is the underlying assumption of "decanic modifiers."

For some perhaps nice color, various druidic traditions would have various lists of spells, and your druid is no doubt proud of the secret knowledge of their own Order. A Perk might give four or five spells they can use this for, while Security Clearance or Rank in the appropriate organization might give them access to knowledge of a full list covering many spells of the Colleges the order focuses on.

2.) Give your Druid's Magery the Trigger limitation at -20% (saving 2 points per level above 0, and another 1 if you also apply it to Magery 0, though I wouldn't): each spell requires a "dose" of some material substance. The materials needed are common, but take some work to find, and may not be available in distant or alien locales. "An herbal preparation, created by a successful use of the Herb Lore skill with a specific spell in mind" would be very suitable for this, and the cost and time should be reduced drastically from the usual making of an elixir since it's not the alchemical preparation doing the spellcasting but the druid themself.

3.) To really sweep a lot of it under the rug, give their Magery the Pact limitation at -10% and require the druid to spend a few hours a week in a natural area: feeding birds in exchange for feathers from the nest, cutting mistletoe off of oak trees with a bronze sickle, etc. Assume their Magery works well for the next week as long as they have access to their usual gewgaws and let the player fluff it. However, the material loses its magical utility quickly and the druid must renew their stock weekly.

Alchemy and enchanted items, which have had a lot of work put into them, would maintain their potency indefinitely and the latter should probably be allowed as a substitute for at least a couple of particular spells -- for example, a wand of Fireball should probably also work as a nonconsumed, permanently available material component for perhaps Ignite Fire and Seek Fire.

Edit to add: that Pact limitation requires a 10-point Vow, which would include things like "protecting nature," "keeping the secrets of the Order," "regularly spending time communing with nature" to cover the materials requirement, and anything else appropriate for the tradition you have in mind.