Looking at the PC case descrition... it says it comes with 1 fan (which Should be at teh back of the PC). These fans are a size of 120mm. As well as the PC case can have 7 fans installed (that's a bit overkill). You could look for something like... 2 kits of 2 pack fans, 120mm. 4extra fans should be plenty, don't need to fill every slot. And for that matter Even 2 would increase the cooling by quite a bit (since i'm unsure of your budget). \
An important note is that there may be a bit of a "theme" in color going on there... the PC case is Black and Red, Rear fan is Red, There should be another fan on the CPU (so kind of in the middle when you look through the window) It might have some lighting as well... if that's red.. then you can go for FULL out red theme fan lol.
I would probably Suggest Corsair SICKLEFLOW fans... just because they are budget friendly and have pretty decent performance. (also it depends on what you get as a better deal, buying 1x or pack of 2). As far as where to buy, Newegg is pretty reliable and easy to navigate.