Try with Coban Travel.
You can just book it when you are there. In Flores San Juan Travel has a bit of a monopoly and they are not always honest so make sure you have a receipt that clearly states what you paid for. Or try Crasborn Travel or Los Amigos hostel (I haven't used them myself but have heard good things about them).
In Lanquin we booked a shuttle through our hotel. I think that is what most people do.
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