you can purchase the bags when you open the frontier gate map point after doing the Fort Ranik mission, i believe. you purchase them from the warmaster, i forget his name. you can get runes of holding in post-sear ascalon or possibly at the merchant at the great northern wall. i'll confirm this, as i'm not 100%.
*edit* to get the bags, you buy them for 100g from
Warmaster Riga at the Frontier Gate. you can buy 2 bags from him and apply both to your character right there, having all your bag slots filled if you already got the belt pouch. you can get the runes of holding at the earliest from
Rolf [merchant] in Great Northern Wall or
Vencel [merchant] at the Sanitarium. the rune costs 500g at either merchant.
i don't know about the signet of capture, because i'm still in the mountains. hopefully i'll get to play with it soon.
hope the info helps