How you are collecting, raw coins vs slabbed coins, will dictate where you can find good coins. The very best resource for finding coins, both raw and slabbed, without paying high retail is to network with other collectors. If there is a coin club in your area joining and being active might lead to finding some of the coins you want at (possibly) slightly lower prices. Plus you have the added benefit of seeing the coins before you buy them. The coin marketplace here on the forum is another resource that you can use to look for coins, it has limited use due to the small number of active participants here. But it is free to use and never hurts to look or place a want to buy thread and see if you get any bites. As there is no real safety net or recourse on the forum please use caution and get references if you are unsure before you send monies, we don't see many but every now and then a scammer does show up.
Auction sites like Heritage and Great Collections are excellent sources of slabbed coins, you will pay retail prices but not the high retail prices that you will from many other places. You may on occasion find a "deal" on Ebay and apmex, but you are more likely to pay full retail or get lesser quality coins. There are any number of good dealers, both national and (sometimes) local, these can often be a source for better quality coins but the flip side is you will most often pay high retail. After all many dealers are buying from the auction sites I noted above and then marking up to make their profit.
Circling back to my first comment, if you are planning to buy raw coins please do your research before buying, there are many very deceptive counterfeits out there. I would strongly suggest that you only consider buying the key dates slabbed, paying thousands of dollars for a twenty dollar fake really stings.