I buy the following only because these
State Quarters have a nice modest mintage:
a) The Uncirculated sets from 2005 - 2010 (this includes the start of the
ATB Quarters). They had a finish different from the business strikes. Only drawback, serious collectors know this and usually do not put them into circulation unless they buy several sets and cherrypicked the best - then the remainder may appear in circulation or on the secondary market.
Otherwise, as more and more people realize that the market of Uncirculated business strike
State Quarters are oversaturated I see more and more being put into circulation. This has accelerated since the Mint issued the W Quarters and collectors also start looking for nice
ATB Quarters in circulation (which is hard as the relief becomes ridiculously low with these Quarters.) So if I find a very fine specimen of a
State Quarter in circulation I keep it. SO far this year only found one that was extremely fine shape (basically a fine BU) and two or three others also in Mint state (or AU58).
What I find harder to find are (well for most years) really nice
ATB Quarters especially from 2010.
State Quarters are not my main focus at all (exception being the satin finish coins and that is more prominent in my goals but still not my main one). I think they are a great coin to train your eye on grading and do research while being quite sanguine about the value of the coin. Any
State Quarter of some significant worth now and in the long run would have to grade above MS 67 ---something that even in Mint sets is not easy to find.
As for finding silver quarters - depends on the price of silver. You can find silver coins but expect to find such coins a few times a year and across different denominations. The easiest to find are the 40% Kennedys - tons were made and the public usually do not know that after 1964 there were some silver in a few coin series until 1970 circulating.