Hi, Meena,
I don't know of any stores personally, but resorting to google produced a couple of options, but they are all a short cab ride out of town:
http://www.lojadamontanha.com/cms.php?id_cms=4 (looks like a serious store, but it's 5 minutes outside Porto, probably not much of a cab fare)
Decathlon, the mega-store you can find in much of Europe, has two sites:
http://www.decathlon.pt/nettools/store/ ... thlon-MAIA
Like the other store, though, they are out of town, in industrial areas. That seems to be part of their business model, to go where the real estate is cheap, because in every town I've ever seen them they are located in some industrial warehouse wasteland.
In the end, I would probably just stick a lightweight bag in my pack at home and carry it with me. There are many bags around a kilo or less.
And that would also allow you to stay in the albergues that are south of Porto -- though there aren't many, there are some -- have you seen the list on the Via Lusitana's web page? People especially like the one before Golega and the one right after Mealhada in Sernadelo.
Bom caminho, Laurie