It sounds like you're new to the building process, and I'm not sure what your tooling situation is but if you don't have the tools, ie.... decent table saw to make consistent cuts and a router table set up with the proper bits, then obviously It would be easier and cheaper for you to buy them pre-machined.

I'm not sure what your future plans of kayak or canoe building are but if you're in it for the long haul (multiple projects) like many of us around here are, then its always good to give machining a try and see how it works for you, plus it gives you an excuse to buy fun tools!

I myself have the tooling and love seeing big pieces of cedar turn into a couple hundred thin strips in the matter of one weekend. I think if you have the tools to do it then by all means do it yourself, you can control what kind of wood goes into making your boat, and you feel like you have more of yourself put into the boat instead of having them done for you.

My first project was a kit through Newfound woodworks and they sent an excellent selection of strips, all of which were machined to perfection (sometimes I can't say the same about my strips)
Try them, I know that their prices are fair and the people are great to deal with. I highly recommend them.

Pat
Pitman, NJ