Yep - it's an unusual subject, but if one travels to the Lucerne area to plan a wedding and a reception: is there a place to order a wedding cake that won't put us into bankruptcy ;-)
Well you picked the wrong locale if $$ is an issue. Switzerland is famously pricey. You might direct this question to your hotel there or whoever you are arranging your accommodations with.
Personally I use google & google maps - I needed to find a hair salon in Barcelona - I will be two months into a trip - with a month to go - and will really need a salon by then. I was able to find one who uses my products, near my hotel, and has excellent reviews.
I do not have any personal experience with this vendor, but they may be able to help you.
Hi again -
Did you try Bachmann? This is a popular local bakery in Lucerne and they also have a website in English with prices http://www.confiserie.ch/en/torten/hochzeitstorten
Another local bakery is Heini but I think their site is only in German http://www.heini.ch/einzigartigkeiten/87-heini-hochzeitstorten.html
If you are looking for something more bespoke/fancier, you could contact Gillian at https://www.gillianscakes.ch/ She is based in Zug (about 20 mins from Luzern) and is from England, so should be easy to communicate with her if you have specific wishes etc. Prices are obviously higher though (not sure of your budget)...