Sometimes in any business it becomes necessary to transition from old faithful products to new ones. This can be stressful and even traumatic for you and your customers when dealing with a comfort food such as tea. When facing these kinds of challenges managing customer expectations is key as is the old adage, "To be forewarned is to be fore armed."
First, communicate as well as possible to your regular customers that change is coming. Explain what items will be affected and why the items they love will no longer be available. Do this in advance of the coming change to give them time to process the information. Explain you are actively looking for new suppliers and hope to get as close as possible to the products they love. They might even have suggestions for you.
Secondly, when the initial wave of concern has passed let them know you have found a new vendor. Talk about the vendor's attributes as a company. Let them know the teas are not identical and what differences you detect. Then lastly, add the new items to the line and let your customers provide feedback. Explain that they might need to try the tea a couple times to allow their palate to acclimate.
Making the customer a partner in the transition will keep them supportive and feeling less alienated. It is not easy to change someone's comfort food! Alas, change is inevitable. As they say, the only constant in life is change, so learning an ideal way to ease into the new and different is not just a great coping skill but also a great business skill.