Every convenience store chain is in all around areas in Tokyo. You won't have problems finding LAWSON in Shinjuku, as well as Family Mart in Shibuya.
- The convenience store chain with the best selection of Oden
All the store looks the same to me (sorry, oden lovers!) but it seems like 7-Eleven has largest variety.
7-Eleven (68 kinds) http://www.sej.co.jp/i/products/allergy/oden/
Family Mart (26 kinds) http://www.family.co.jp/goods/ff/oden/
LAWSON (47 kinds) http://www.lawson.co.jp/recommend/original/oden/
Each store has slightly different taste of dashi, it maybe interesting to compare :)
Many of the food in convenience store use preservatives in some way. However, they are shifting to avoid using such materials. It may change in few years.