Here's an excerpt from their website :

In your country, we do not currently issue Ukash directly from stores. However, we do have a number of globally authorised resellers where you may be able to obtain Ukash. Check the latest list below.

So I don't believe you can get them at any store here.

There are some global resellers you can purchase those vouchers from. Here's a link from their website containing a list of those resellers.

Hope that answers your question!