Hi Kimberly,
Good thing you didn't ask what brand of roof rack to get. This is guaranteed to spark a holy war.
You didn't mention what type of car or kayak you have. If it's a whitewater kayak, and you have the 60/40 fold-down rear seats in your car, then you can easily stow it inside. (I've seen a guy with a toyota doing this.) If you have a sea kayak, then roof mounting is your only option (unless you want to tow a trailer).
Anyhow, by nylon tension tie-straps, I take it you mean the flat nylon (or whatever material it's made from) with the self-gripping cam buckle.
You can get these straps and the foam blocks from any of the stores in Westboro like, MEC, Trailhead, Bushtukah, etc. I think I've seen them at Canadian Tire as well.
One problem with this basic setup is that you'll need someone to help hold the foam blocks down so that they don't blow away with the wind. Otherwise you can use some double-sided tape or any tape to temporarily hold the blocks in place.
I've tried having the basic foam blocks and straps with my current rain-gutterless sedan. While this 'works', one thing to consider with this setup is that when you put the centre strap around the kayak, it will pinch your car door rain seals and it WILL leak.
I did this once, just before a thunderstorm. Suffice to say, my back seat was a little wet.
Another thing about having just the blocks, you have to watch that when tightening the straps you don't deform your roof. (Yeah, I did that too).
As for what I use, I went at bought myself a roof rack for my car from Auto Racks in Westboro after that thunderstorm. But, I still use the foam blocks.
Hope this helps.
Cheers,