Just by the sheer size of the cable industry you can see that there's a quite a bunch of people that think cables matter a lot for audio, but then there are scientific studies that prove that there's no audible difference if you're using the right gauge for the right distance.
If you want them custom made, I hope you mean making them yourself? just buy proper Neutrik connectors and some decently thick cable and solder it together.
buying in store is no problem, you don't get the exact length you might need but it is easier. A brand like Klotz offers great cables at decent prices.
Personally I never spend more than 15 euros on 1.5 meter of RCA cable and I think thats already quite expensive. For XLR I buy Tasker cable, they have Neutrik connectors and quality cable and are decently priced. 15 euros for one 3m XLR male-female cable
Technics SL1210mk2s and a Freevox DJ CLUB