I tend to buy my parts online from stores outside Quebec to avoid paying provincial sales tax (~10%). Shipping prices are quite low most of the time and delivery times to Montreal are short enough in my experience that shopping at brick-and-mortar stores isn't all that useful.

My goto website is PCPartPicker to browse for items sold across multiple stores and compare each store's prices.

As far as individual stores are concerned, I've ordered numerous times from NCIX, MemoryExpress and Newegg that I can recommend buying components from them. Do make full use of price matching policies to save a bit more if you want.

Here's a video by Justin Bender giving you step by step instructions: https://www.youtube.com/watch?v=DqORu39gktw

Like the video, I'd also suggest you use a marketable limit order.

And there is no reason not to buy everything at once. If you are not comfortable with this maybe it's a sign that your risk tolerance isn't as high as you think it is.