1 x sliding door and 1 x sliding window

Hi Guys,

I'm looking at making some new openings for a door and window in the new year, we have contacted two glazing company's but they don't do the exact style we are looking for but we have found it online( for cheaper than the glazing company's option) however I cannot seem to find anyone who advertises just install service- they all seem to be supply and fit.

Anyone just had someone just install? I'm guessing they don't like to as it could be the wrong size and they don't want the blame or have you had someone measure - you order then they fit? At this rate I'm tempted to just pay the building control charge and fit them myself but never done it before