Advice on Air Con units please (Montréal)

Hi all, I have never had to buy an air conditioning unit and I've realised our condo is getting very warm already and its not even peak summer yet!
Any advice on where to buy/ buy second hand? What to avoid.
We are on the plateau with those windows that slide up so I assume we will have to get one of those square ones you slide into the window?

Many Thanks in advance. Have only just moved here so not familiar with any shops, brands etc.

Anita