I'm an *ahem* 'larger lady' and my trusty and rather ancient sports bra recently collapsed under the strain. I didn't have the chance to get to a sports shop so whilst doing the shopping in Asda I had a quick look in their sports section, there was a high-impact, front-zip, racer-back sports bra for £6. My immediate thought was that it'd be rubbish but I picked one up anyway.
I tried it out today and it was great - really supportive, very comfortable, no riding up at the back or digging in on the shoulders and very little bouncing
Unless it falls apart in the wash it's a bit of a bargain!