I'm a 28 back too, but a G, and it's a difficult size to source. You can probably get away with a 30, but it won't fit as well.
There is a site called The Little Bra Company that stocks a 28C, but that is American.
Marks and Spencer's do a 28C - I think it might be their teenage girls range (which isn't as bad as it sounds) but also in their Limited Collection.
I think (though I may be wrong), Freya have a 28C.
If you need a sports bra, Shock Absorber do a 28C. I have a 28G and it is awesome - totally unshifting as you run.
With a size like yours, you won't find a lot on the high street I'm afraid. eBay is your friend, also try figleaves.com.
Of course, if a 30D fits well, that size is pretty much EVERYWHERE!