go to
www.stamps2go com
Hundreds of sellers with piles of stamps, all listed individually for reasonable prices. Strongest in US but also worldwide. Shipping for any amount from a single seller is always only $1.00. The betters are usually accompanied by a scan but the commmon stamps may not--you have to go by the f/vf/mint/used descriptions and trust the seller.
Take two or three of the ones you think will be harder but not too hard to find. Search for them and note which sellers are offering them. Put two or three examples of one stamp into your shopping cart. Once you have one item from a seller in your shopping cart, you can then search for all the other Scott numbers on offer from that seller only. You can fairly quickly get a sense of which two or three sellers are going to have most of what you want and in condition and at prices you like. Then make up an order of twenty or so, shipping only $1.00.
Not a lot more time consuming than browsing through a bricks-and-mortar dealers counter cards or counter-books, especially after your first session or two. Lacks all the fancy-dancy
www.ebay.com/b/260/" rel="nofollow">ebay watch list, feedback etc. rigamarole.
But do keep in mind that when you put something into your shopping cart, it disappears from the on-offer database. So you can keep it in your shopping cart (making off limits to other browsers) for 12 hours (maybe it's 24 hours now?) but if you don't go ahead and purchase within that time period, it will be removed from your cart. That's good in the sense that you have dibs on it, it's bad in that you can't let stuff sit there for weeks but need to fish or cut bait. So don't put something into your cart unless you are ready to make a decision on it within hours.
A good way to buy common to moderately better stamps whose catalogue numbers you know. Not a good way to buy stamps if all you want to do is browse around.
Probably not a good way to buy high-end stuff. And if you only have a relatively few holes to fill, then it may be that stamps2go is not for you. On the other hand, it sounds like you want to fill holes in the 20thc US sections, so perhaps you aren't only after the really expensive ones. Of course, if the holes you are filling are WF coils and their ilk, well, then Stamps2Go is not the right venue for you.
Edited: don't know what the shipping cost to Canada is, probably not as low as $1.00 but probably not bad.
Stamps2Go is a cheaper version of the APS StampStore or Zillions of Stamps.