It looks like you will be travelling on a Canadian passport? If so you have several options. The more rock solid one would be to purchase a multiple entry Thai visa at the Embassy in Ottawa. You will still need to purchase visas for the other countries you are visiting but I think you can do that at the boarder. A second option would be to do your Thai entries on a visa exempt entry. You might get a little more concern from Thai immigration but the only for sure rule is no more than 2 land entry visa exempt entries in a 12 month period. As always when entering Thailand have
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baht in cash or the equal in say CAD, proof of onward travel, and proof of a place to stay.