Hey all,
I am pretty new to cycling so I apologize if this seems like a very basic question but I am wondering about upgrading my components. I currently have Sora shifters with tiagra derailleurs and that are 3x9 speed. I would like to upgrade them to 105 shifters with ultegra derailleurs. Aside from these three parts, what else would I need for this upgrade? Where would be a good place to get the shifters and derailleurs cheap (even lightly used)? Thanks.

Do you want to end up with 2x10 speed or what?

I'd like to keep the same crank so not really unless its not possible otherwise.

You need a 10 speed chain, 10 speed Cassette. I upgraded from 9 speed to 10 speed. I purchased all my components from Amazon.

I would change the crankset to a 105 triple along with changing to Shimano for-triple brifters, derailleurs and the rear cassette.

You can mix a few the 9 speed and 10 speed componients, but the result is never perfect.

The crankset is where all the weight savings is anyway. If you want to dump the Sora shifters for better ones, that'd be a good idea. I wouldn't waste money changing the derailleurs (well maybe the rear if your current one will not shift all 10 gears, which it probably will be fine). Get yourself a 10 speed cassette and the two brifters and some bar tape and have at it. (and yes, the chain too, but others beat me to it).

To OP, your speeds on the cassette and crankset need to match the speeds on the shifters. The deraileurs don't care. If you go to 10 speed you will need a 10 speed chain.

So if you are getting 10 speed 105 shifters you will need STI-5703, a 10 speed cassette and a 10 speed chain. I would leave the derailleurs alone.

Sounds good thanks so much everyone. I may need to replace the rear derailleur anyway, if this is the case would it be worth it to upgrade or get a 10 speed too or would it not make a difference if I just get the same one I have? I guess I will look into a new crank now too...

It may not matter, but if you do replace the RD i'd get a current model just to keep the group consistent. But the main thing is to be sure you have an rd with capacity to accommodate your gearing. Triple cranks and wide range cassettes can require mid or long cage rear derailleurs.

Or you can just upgrade to 9 speed Tiagra shifters (4500 series) and get all the benefits of the 105 with no other changes necessary.