CNL is a subsidiary of Deutsche Bahn, so the best place to book is generally bahn.de. The starting price for 1 person should be €129 (~$170) in a 2-berth deluxe sleeper, or €109 (~$145) in a 3-berth deluxe sleeper. (Note that these are buy-in-advance Europa-Spezial fares, so price increases as availability decreases, no changes allowed, and cancellations are at a fee (of I think ~€45)). Self-print online tickets should be available for this journey (but note that, unlike airline e-tickets, you must have an A4 print-out of the ticket with you).

The full-fare price is €234 for 1 person in a 2-berth, which seems to be roughly what RailEurope is charging.

Although it seems that 3-berth compartments should be available on this train, for some reason bahn.de won't sell these, so if you want a 3-berth, you'd need to call DB (+49 (0) 180 5 996633). They would then either post you the tickets, or give you a reference number to pick up the tickets from any DB self-service ticket machine (of which there are plenty at Munich Hbf, so you could pick the tickets up shortly before departure). However, if you want a combination of 2-berth or 1-berth compartments, then this can be booked online.

Note that ticket sales only open 92 days before departure. (You can actually make reservations on the CNL 180 days before departure, but not online, and this would presumably work out more expensive, as you'd have to purchase the sleeper reservation and the ticket separately).

For more information about the CNL trains are actually like, see Seat61.com.