Toiletries / Cosmetics
Chiang Mai has many shopping malls that offer the same products as outlets in the US.
Electronics
The voltage system in Thailand is different. You will need to use an adapter for any electronic goods that you bring. Check online for comments from others if you are unsure about the differences.
Cell phones: you should contact your cell phone service provider if you can use your phone here and if there are any additional charges. Some students purchase a Thai SIM card for local communications.
Bring your phone charger!
Other
Other products like mosquito repellent you can buy here.