If you want to go with used, but 99% likely to be in great condition and ready to go out of the box, then the classifieds on this site are the way to go.
If you want to go with new in the US, then Straight Razor Designs is the place to go.
If you want to go with new in Canada, then I defer to RezDog above.

I don't know how much you want to spend, but most decent new straight razors start around $100 (USD) and go up from there.
That is a warning to basically stay away from the $15 dollar things "advertised as razors" on amazon and ebay. You don't want to buy one of those.