As everyone says Dan, for most dates, the coins can be found. However all of us here (well, pretty much all!) have to make a decision at some point about balancing what we'd like with budget. Which in turn affects the speed at which your collection will grow.
Personally I seem to buy less than half a dozen coins a year. Four so far this year, though it was only two in 2007. That's a result of my only wanting certain coins, being particular about what condition I want and how much I have available to spend.
Some people might find that rate of building a collection too slow. I did myself a while back and diversified for a bit because money was burning a hole in my pocket! Now I'm back on track with my main collecting interest but funds are low. Fortunately (?) coins that appeal are also in short supply!
I guess what I'm saying is that you'll need to decide at what point you are prepared to accept a coin that, were it a more modern issue, wouldn't make the grade. Pun intended.