I think you are right, breadmobil, it is a custom build. The parts used are known as 'System X' and they fit together with those small red and yellow connectors. If you are not familiar with System X buildings, you could get hold of a secondhand one from eBay and play around with assembling it in different ways. Juve-bil will be able to tell you the origin of specific pieces, but to aid your impatience, you could look at PlaymoDB.org (forgive me if you know all this by the way, but you did say you didn't know where to start!).
On the homepage of PlaymoDB.org, under 'Parts' there is a drop-down list next to 'Browse parts in a category of your choice.' On the list, choose 'Building' and you get a page showing about 20 categories of building parts. The top one (General) and some of the ones called 'SystemX' are probably most promising. Clicking on one of those will bring up a list of parts, with pictures. Clicking on any part will take you to its parts page, showing which sets it came in.
To get hold of a specific part, you could search on eBay (for the set it came in, or the specific part), or buy it new from Playmobil. Only parts used in recent sets (up to about 8 years old) are available new (and not always then) and only if you know the 8-digit part number starting 30...... , (which you got from PlaymoDB). Go to playmobil.com, choose your country at the bottom of the home page and then select 'spare parts' for ordering parts.
Post a question on here for more guidance, or help with sourcing particular parts.
