Welcome to Set Building 104. In Set Building 101, you learned how to obtain cards, and in 102 you learned how to sort and collate those cards into numerical order. In SB103, you created want lists. Now, here's how to obtain the other cards you need to finish your set.
One of the greatest ways to complete sets is through trades. However, this only works if you're collecting sets that others have. Many sets are perfect for trading, but for more unique sets have to be found through other means.
If you're collecting base sets, many singles can be found on Sportlots.
For inserts, autographs, and relics, many people turn to Check Out My Cards. The site is also good for more hard-to-find singles.
While many people don't like eBay, there are still some great finds to be had:
2011 Topps Heritage/1962 Topps Buyback
2011 Topps Heritage Green Refractor
2011 Topps Tribute #/199.
All three cards came from the same seller at a fairly good combined price and goes towards the sampler collection.
That completes your course in Set Building 104 and last Thursday's mail day! Yes, I'm that far behind.
And by the way, obviously I'm half-awake when I write some of these trade/purchase posts. Sorry for that. I like to play show-and-tell, but sometimes I'm writing them from memory.