Primark
As I spend my whole clothing allowance (ha! ha) on my horses, I get my sports bras from Primark and find them the best! I think they are £5 and they last about 6 months of daily wear and constant washing, then I throw them away and start again. They keep me still and comfy! I also recommend their seam free pants as well for under jods, absolutely comfy and no horrible cut in marks when I get off!!
The Sports Bras are quite wide straps - really are over shoulder boulder..... - but do the job, eg not pretty, but I have seen much worse for much more money.