Help with going from Chiang Rai to Laos

Need help finding a budget way (doesn’t have to be the absolute cheapest) for a couple to get from Chiang Rai to either Luang Prabang or Vientiane. Don’t care which one, just whatever is cheaper/less of a hassle in terms of the journey.

Willing to take long bus ride but would want something not too uncomfortable. Can I book online or do you have to buy these tickets in person? Hoping to use a bus ticket as proof of onward travel when entering thailand. Any suggestions on websites or specific bus stations etc?

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