Short answer: you can buy anxiety‑support foods and calming supplements for cats from your veterinarian (best for prescription/therapeutic diets) and from major pet retailers online or in‑store (Chewy, Petco, PetSmart, Amazon, Walmart) and from veterinary supply sites. Below are concrete product examples, where to buy them, and quick buying tips.

Where to buy (with example products)

• From your veterinarian / clinic — best route for veterinary‑exclusive therapeutic diets (e.g., Royal Canin Veterinary Diet Feline Calm). These diets are intended to be used under vet direction and are often sold only through clinics or require vet authorization. (royalcanin.com)
• Big online/brick‑and‑mortar pet retailers — Chewy, Petco, PetSmart, Amazon, Walmart sell many calming supplements and some vet diets (some vet diets still require vet authorization). Example: Royal Canin Calm is sold through major retailers but is a veterinary formula and may require authorization. (petsmart.com)
• Over‑the‑counter calming supplements and chews (no prescription) — widely available online and in stores:
  • Purina Pro Plan Veterinary Supplements “Calming Care” (probiotic calming topper/sachet) — sold directly by Purina and through retailers. No prescription required. (purina.com)
  • VetriScience Composure calming chews for cats — sold at Amazon/Walmart and pet stores. (amazon.com)
  • Nutramax/Solliquin calming chews — available at Petco and other pet retailers. (petco.com)
  • Vetoquinol Zylkene (calming supplement) — sold online (e.g., Amazon, vet suppliers). (amazon.com)

Other places to look

• Veterinary pharmacies and online vet supply stores (Lambert, BaxterBoo, farm & fleet Rx) — often stock both supplements and prescription diets. (lambertvetsupply.com)
• Local independent pet stores — good for getting advice and quick pickup.
• Pharmacies or big‑box stores (Walmart, some Targets) — may carry popular calming chews and diffusers.

Quick buying & safety tips

• Prescription vs. OTC: therapeutic “calm” diets (Royal Canin Calm and similar) are veterinary‑exclusive — talk to your vet about whether one’s appropriate and how to get it. Supplements/chews (Purina Calming Care, Composure, Solliquin, Zylkene) are usually sold without a prescription. (royalcanin.com)
• Ask your vet first: vets can help pick the right product for your cat’s health, rule out medical causes of anxiety, and advise about interactions or contraindications. Professional guidance is especially important for therapeutic diets and if your cat has other illnesses. (AVMA / AAFCO guidance supports veterinary oversight for therapeutic diets and recommends consulting your veterinarian about supplements.) (avma.org)
• Look for reputable brands and third‑party quality seals (e.g., NASC membership for supplements) and buy from established sellers with good reviews and return/refund policies. (purina.com)
• If a product requires vet authorization when buying online, follow the retailer’s vet‑approval process (many retailers contact your clinic or let you upload a prescription). (petsmart.com)
• Try environmental strategies alongside nutrition/supplements: pheromone diffusers (Feliway), enrichment, predictable routines, hiding/vertical space — these often help reduce anxiety and work well combined with calming foods or supplements. (thesprucepets.com)

If you want, I can:

• Look up which of these products are in stock at a specific retailer near you (I can check local Petco/PetSmart/Chewy/Amazon availability), or
• Compare 2–3 specific products (ingredients, price, how long until they work), or
• Draft questions to ask your veterinarian about starting a calming diet or supplement.

Which would you like me to do next?