Zyban is one of the most prescribed drugs for depression in the USA under the brand name Wellbutrin, (manufactured by GSK).
It is expensive and not prescribed under primary care in the UK.
In some cases it can be prescribed under secondary care (psychiatric practitioner), however this will require a referral from your GP once he/she is satisfied all the other options have been explored, which usually means trying a handful of (much cheaper) SSRI medications such as Fluoxetine, Sertralie, Venlafaxine, Mirtazapie etc.
It is a highly sought after drug in the UK as it is an atypical antidepressant that focuses on increasing dopamine rather than serotonin and therefore does not usually bring with it the often frequent and undesirable side effects of SSRIs relating to sexual dysfunction. It's appetite suppressing characteristics are often seen as an added bonus to some.
There are some dodgy on-line websites that proclaim to be able to supply without a prescription from Turkey etc, but you'll pay your money and take your chances.
The best chance of getting it on prescription in the UK is to seek a referral from your GP for secondary psychiatric assessment then request it via that avenue, assuming your symptoms and experiences of SSRI treatment warrant exploring this avenue.
Hope that helps.