Chiangmai to Laos

Hey guys!
Me and my girlfriend are planning a shorter (6weeks) backpacking trip through South East Asia starting in Thailand and moving to Laos from there. The original plan was to visit chiangmai and from there take the bus to Luanprabang.

However I cannot seem to find tickets for the bus available online anywhere. Are these routes no longer operational?
(due to visa limitations we have to show a ticket for exiting Thailand upon arrival so I hoped to buy one online)

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