My underbust is 29 and I buy bras in 32C. I also have a 30 band bra, but I prefer 32. If your underbust is 26 only you could search for 28 or 30 band bras @aliexpress.
Some examples: http://www.aliexpress.com/item/Hot-sale-...45786.html
http://www.aliexpress.com/item/Free-Ship...83558.html
http://www.aliexpress.com/item/KOTY-2014...22652.html
http://www.aliexpress.com/item/new-2014-...74253.html
http://www.aliexpress.com/item/2014-New-...52924.html
http://www.aliexpress.com/item/New-Sexy-...71920.html
http://www.aliexpress.com/item/2014-New-...30504.html
Some examples: http://www.aliexpress.com/item/Hot-sale-...45786.html
http://www.aliexpress.com/item/Free-Ship...83558.html
http://www.aliexpress.com/item/KOTY-2014...22652.html
http://www.aliexpress.com/item/new-2014-...74253.html
http://www.aliexpress.com/item/2014-New-...52924.html
http://www.aliexpress.com/item/New-Sexy-...71920.html
http://www.aliexpress.com/item/2014-New-...30504.html