I haven't noticed you here before so let me start off by saying, Welcome to the Treasurenet Forum.The best site on the web.Gypsyheart and mwsvector have given good advice,I will try to give a little myself.For the most part you will find the best "bargains" at yard/garage sales,estate sales,estate auctions and local flea markets.Many thrift stores sort through the good stuff and price it accordingly or keep it for themselves,which is highly unethical but it happens all the time.There is always that exception to the rule though.Some of my best finds at thrift stores would have to be a book,"The Man in Black" by Johnny Cash,an Elvis musical decanter from McCormick distillery,still sealed and full with contents (bourbon), a couple of "springbok" puzzles from the sixties,a miami dolphins helmet phone from the '80s,many old post cards,some bakelite utensils,swizzle sticks and local (state) vintage art pottery or "tourist pottery" as it is sometimes referred.
I look for the unusual or unique and of course the cheapest I can get away with.Just remember,condition,condition,condition.Original packaging also helps if applicable.For the most part,I buy whatever happens to catch my eye or sparks my interest that day.Buy what you like and you can't go wrong.I have sold many items and I have held on to many more.Certain things will always hold a value and many things will go up in value as the years go by also.
As you mentioned, I would invest in some books on collectibles and antiques,price guides.The "Kovels" put out some good books as does "Antique Trader".There are many out there to choose from,this was just a couple that came to mind.You can always go category specific too,you know,tootsie toys,California pottery,lighters-tobacciana,matchbox cars,marbles,depression glass,glassware,antique writing instruments,advertising collectibles,snow domes,griswold&wagner,kitchen antiques,compacts,occupied Japan,pez dispensers,star trek,Elvis,post cards,records,American art pottery and the list goes on and on.Good luck on your searches and let us know if you need any further help with anything.
Peace-diggummup