I am so sorry for your loos, it must have been such a terable shock for you. Unfortunatly baby buns are delicate things and many pet shops do not treat them very well. Perhaps he was scared being on his own and the shock of moving but it is likley he was ill before he came to you. Please don't beat yourself up thinking it was your fault.
Take some time, take a deep breath and then, if you still want to have a rabbit companion approach a good local rescue for help. if you dont know where to find a rescue dont worry we can help there. The best part about a rescue bun, other than knowing that you have helped it, is being able to call the rescue and ask advice and for help if you need it.
Most people on this board have suffered a tragerdy and we do sympathise with you.
If you felt up to it you could also tell the local council or even the RSPCA so they can check and make sure the bunnys at that pet shop are being well looked after.