You will find that all good spray tan companies will sell pop up tents. Just look in the distributor section of here and you will get a list of spray tan companies. Alternetively do a search on google for uk spray tan companies.
It is better to purchase from a reputable company so at least if you have a problem or something happens further down the line you have the support and back up from who you purchased it from.
Good luck if you have any problems finding one please feel free to pm me.