28C is indeed the hardest size to find. 28 and 30 A and B are common for petite bra brands, 28 bands and above in D cups and above are common for full bust brands, 30C is common for mid sized bra brands, yet 28C is commonly ignored, even though I think that 28C occurs more commonly than 28A, 28B, and 30A.
Here are some suggestions for finding a bra in 28C:
5. Skarlett Blue is a relatively new brand, having been on the market only since Winter 2014. They cover a sorely missed spot in the lingerie market: the small-band medium-cup size range. They carry 28C - 30E, 32A - 36F and have just one style in 28C: the Minx Front Close Racerback Push-up bra. https://www.skarlettblue.com/bras/minx-front-close-push-up-plunge