I always go to my local used-CD store, they have a deal where you can get 3 discs for $9.99. Unfortunately, they don't get new Broadway stuff so I bought all I need from their collection.
I suggest half.com, amazon.com, etc. I limit myself. I buy a couple once a month. If I know there's a new cast recording coming out that I REALLY want, then I just pay full price and just buy one that month.
Really, you have plenty of time to complete your collection. It's a lifetime activity, you don't need to have them all now. It's really fulfilling to see your collection grow as time passes by.
Also, sh-k-boom.com has some brilliant sales sometimes.
"Some people can thrive and bloom living life in a living room, that's perfect for some people of one hundred and five. But I at least gotta try, when I think of all the sights that I gotta see, all the places I gotta play, all the things that I gotta be at"