Update: Searching for individual crate series has been added to the steam marketplace. You can now search for any crate series by selecting crates and then entering # and the crate series number. On the downside, you won't find any deals for unboxing stranges, most of the desirable crates are very expensive (Check out #30). You might as well trade for them frankly, unless you like high-stakes gambling.
Searching for individual crates series is often futile because a majority of items in crates are worth less than the price of a key (which is worth at the the time of writing about 3.33 ref). While I have no direct evidence to support the claim, I believe that most valuable crates are snatched up quickly by savvy traders.
The Naughty Winter Crate 2012s with the festive items are a good example of this. Strange festive weapons are all worth more than the value of a key, and thus opening such a crate is a no-brainer. A trader will almost certainly profit from opening such a crate.
To look for individual strange weapons, I first go to backpack.tf - Price List, to find the value of an item. Then I use TF2Outpost or TF2Bazaar to search for sellers of a particular item. I then look through the listing in order of newest first, and look for people who are selling individual items rather than whole sets. Then I usually add them as a friend and wait to see if they respond correctly to the friend request. If they don't respond in 30 seconds, I move on to the next seller. I have about a 40% success rate of getting what I want after a friend request.
If one wishes to bargain one can usually get a better price on something, but many high-volume traders will refuse to bargain. One can often search the whole listings for a deal, but one may have to wait for that trader to come on line. Because I want to trade quickly, I occasionally pay slightly more than listed value.
If one is totally out of metal, then one can buy a key and trade that for metal using the same sites. Certain rare weapons and hats can be traded for keys, metal, or ear-buds.
Strange Festive Rocket launcher, strange Kritzkrieg, and strange gunslinger are some of the most valuable items at the time of this writing. The 2011 Halloween hats crafted from the weird metal of that event are worth a lot.
Also note, some items are not very valuable but they sell quickly so they can still be difficult to obtain. Consider paying more if you are having trouble getting the item at the listed price.
