Bike parts are largely interchangeable but you do need to know the specifics (shifters have to match rear and sometimes front derailleurs, bottom bracket has to match cranks, diameters of bars, stems, seat posts have to fit each other or the frame, etc.)I like my Trek and have had it almost a year, and have put about 3,000 miles on it.
My Question: Can I get parts cheaper elsewhere? Are there any parts that have to purchased at Trek since it is a Trek Bike?
Yup, especially true for "the ratio on the larges ring". I just googled that bike and it comes with a compact. So you can't just slap on 53/39 rings.While you can get these parts (or all the parts) from many many online stores or somewhere like ebay, and these places will be most likely the cheapest, you really need to know exactly what part you want and then the knowhow and tools to install it.
Best bet would be to wander on down to your nearest biek store (or try a few till you find a store you like to deal with) and see how much they can do it for you (get the correct parts, advice on which parts you might actually want, and then install them for you).
Or buy a book on bike maintenance, and go online buying crazy for parts and tools.
1.2 is Sora.No, I am assuming it's a Shimano 105 cassette? You would have to get another gear ratio cassette from Shimano, which is sold at any online bike store. Or you can pay more at your Local Bike Shop(LBS) and get it instantly, or get it on Amazon or eBay. I brought another Sram cassette difference than the one that came with my Cervelo S2 on eBay likely used because I wanted an option of different gear ratio like you.
Just make your your rear derailleur can accept your new cassette ratio.