So far I’ve only seen sardines in city super at prices of 200nt and up. Couldnt find them at Welcome/Matsusei or any other supermarket. I saw that in the past there was a company called YIwei food that had them for 120nt, but unfortunately they dont have a website anymore.
As for Herring, I havent seen it at all in TW.
Any help will be appreciated
try this
www.pnpfood.com/ - Translate this page
2011年12月27日 – No.9-14 Kin Man St. Taipei 100 Taiwan R.O.C. TEL: (02)2365-0633 FAX: (02)2365-3320. Office Address: 100 台北市中正區汀州路二段189號 … 
Filipino stores carry Spanish sardines at 60nts. It’s not cheapie but OK.
rollmops mentioned by jdsmith are the ticket. Picked herring, and I’m not used to the skin attached but as good as I’ve found here.