While it doesn't come anywhere close to your "reasonable size" requirement (the smallest size available for most types is a half pound), Ken's Fish does offer several spirulina flakes, most of which mention Tropheus specifically in the description, and one of which claims to be 100% spirulina. There's also contact information on the homepage where you can get in touch with him if you want an exact percentage of spirulina content.
I've ordered from him before, though not for Tropheus food, and was quite satisfied with the product.
http://www.kensfish.com/kensflake1.html KENS PREMIUM SPIRULINA FLAKE:
CICHLIDS, ANGELS, AND SALTWATER VARIETIES ABSOLUTELY THRIVE ON THIS VERY SPECIAL FOOD. CONTAINS EXTRA SPIRULINA ALGEA!
PURE SPIRULINA FLAKE:
THIS IS A 100% PURE SPIRULINA FLAKE THAT IS EXCELLENT FOR TROPHEUS.THIS IS THE PUREST SPIRULINA FLAKE ON THE MARKET.EVEN THE BEST SPIRULINA FLAKE CONTAIN ONLY 3%-14% SPIRULINA DUE TO THE COST. THE REST OF THE GREEN COLOR COMES FROM DYES. YOU WON'T HAVE TO WORRY ABOUT BLOAT WITH THIS FLAKE BECAUSE IT IS NOT FULL OF FILLERS LIKE OTHER. ONCE YOU TRY THIS YOU WON'T FIND A MORE PURE FLAKE ON THE MARKET.EXCELLENT TO BRING OUT THE COLOR IN YOUR FISH AS WELL BECAUSE OF ITS PURE STATE!
KENS PREMIUM TROPHEUS LOVERS FLAKE:
THIS IS REMARKABLE FORTIFIED FOOD THAT WILL KEEP YOUR TROPHEUS AND OTHER FISH IN TOP FORM.THIS CONTAINS OVER 20% MORE SPIRULINA THEN MY ORIGINAL SPIRULINA FLAKE!
KENS PREMIUM SPIRULINA + FLAKE WITH GARLIC & PAPRIKA:
THIS IS REMARKABLE FORTIFIED FOOD THAT WILL KEEP YOUR TROPHEUS AND OTHER FISH IN TOP FORM.THIS CONTAINS A HIGH AMOUNT OF SPIRULINA AND THE ADDED INGREDIENT OF GARLIC & PAPRIKA!