If you are looking to hold a simple gathering where you need to buy decorated disposable plates and cups, you can check out MrDIY.
A set of 20 paper cups are priced between 23 to 40 baht depending on the design. Some can be used as cupcake holders. In my workplace in the past where we used to hold gatherings a few times a month- either to celebrate someone’s birthday, team get-together or someone’s farewell, we have opted to buy disposable cups, plates because no one have time to wash up after that. At times it is nice that someone would bring along cheerful sets like what you see above.
Even though the options available are not extensive, ie if you wish to get something like costumes or pinatas you would need to go to specialised party supply stores in order to buy or rent them.
Else, MrDIY would carry the general decoration items at a relatively affordable pricing.
They do sell ribbons, wrapping paper, gift boxes and gift paper bags for gift giving.
I do see that they also sell wrapping paper both in single piece and in bulk. Hence if you are buying presents for someone, you can get the gift bags, ribbons and wrapping paper from here to be used to wrap your gifts.