www.bluenile.ca (online, very popular and good value)
www.brilliantearth.com (My financee is a enviro girl, so she liked them. a bit more than blue nile, but diamonds are Canadian and she likes how they give a portion of their proceeds to maintain environmental sustainability to the Canadian Northern communities the diamonds are mined from)
www.abdiamond.ca/ (on 17th ave, diamond wholesaler. Very good customer service, diamond dealers)
A diamond purchaser/wholesaler can be cheaper than getting it from a commission store. Diamonds are traded at an international standard, so the base price shouldn't change very much. Several vendors confirmed that their profit margin on a diamond is between 10-15%.
In terms of coverage, if you buy it from a store, they generally sell warrenty and insurance for the diamond/ring. Online stores don't usually have such things. However, it is recommended that you get the gem independently appraised. Blue Nile and Brilliant Earth have a trade up program though, so if in 5 years you wish to trade up, they will take your diamond and give you a new one for the different in price (gems generally don't depreciate in value much).
Do your homework, especially with online dealers. The ones I listed are excellent, but you still want to learn the 4 Cs so you can properly choose what you want. Don't be afraid to buy the diamond and ring separately.
My 2 cents, cut > caret is you have too choose (ie more shiny vs physically heavier stone)