Which charger to get fast charging of iPhone XR?
However, seems that the provided 5 W charger is not able to do that
  [charge the battery to 50% in 30 minutes]. I've been in the Apple
  Store and they said I need an iPad charger, but which one?


You need a charger that outputs at least 18 watts or is rated for 5V at 3.6A (W = V * A)

However, (IMO) you should get a charger that's capable of powering/charging all (not simultaneously, obviously) your devices.  The way chargers work is that the rating 12W, 18W, 80W is the maximum amount of power that it will deliver not the amount of power it will "push" to the device.

So, if a device says it's rated at 18W, it will draw no more than that.  If your power supply is greater than or equal to 18W, it will work.  If it's less, it will either charge slowly or you could damage the power adapter.  See this answer for more details

This is why you get a power adapter greater than what you need.  Getting an 80W, for example, will charge your MacBook Pro, your iPad, and you iPhone, whereas the 5W charger will only reliably charge your iPhone.