To be completely honest, I don’t know.
I know the Winter Soldier hoodie was available online from the ReedPOP Supply Co. during C2E2, but it doesn’t look like they’re involved with SDCC. And the Marvel Shop eventually added most of SDCC exclusive shirts to their site. I’d be surprised if they aren’t eventually made available for online purchase. My best guess is that the earliest they would be made available would be October (in time for Halloween).