If you are building up a bike from a frameset, or if you are replacing components on a bike, what are the best sources you've found?

LBS?

Online - Nashbar, JensonUSA, Merlin, Chain Reaction Cycles, Bike Wagon, Amazon.

Merlin, Ribble, and Race View Cycles I've all had good experience with. They are all UK, but with fast shipping and much less expensive than US based shops.

eBay if you know what you're doing and can give in a good bit of time to bidding

Pro Bike Kit as well since it hasn't been mentioned yet and routinely has decent sales. UK based but OK shipping to US.

Also, Performance Bike and Niagara Cycle Works.

Excel Sports occasionally puts groupsets on sale, but not as cheap as overseas.

It all depends on your goals. Or local bike co-op has some good deals on miscellaneous components too. But, most of them are bottom quality parts. The Portland co-ops actually have a few nicer parts, but prices are higher too.

Google searches and Amazon are also resources to use.

Plus you can turn around and sell anything you change your mind on...

If you're in a hurry, ProBikeKit and the other UK shops already mentioned.

If you have patience and know what you want to build over time, then ebay and craig's list take offs can be a great bargain.

Oh, forgot, Planet Cyclery is in the US and is only a bit more expensive on some things that the UK shops.

I haven't checked them in a while, but just did and noticed that they now sell a 6800 groupset for $560 (excluding BB). That's a big change from when I last shopped for Groupsets, where nobody in the USA came within $200 of the UK prices for 6800.

Merlin and Ribble seem to consistently have the best prices. Then there is Amazon and eBay.
Those are my first destinations then I'll do a Google search.
If you are on Facebook look up Online Swap Meet. I just found a barely used Selle SMP Glider for $85.00 on there.

I built my current 2010 CAAD-9 up with new or damn near new Ultegra, FSA, and 105 for about $700 going this route. Couple hiccups along the way but that's what made it fun .

Exactly!

One of the best parts of eBay is NOS of last years components. When I was building up my Curtlo 10-speed stuff had just come out. I bought a NIB Dura Ace 7703 crankset complete w/ BB for $125 shipped; pretty hard to beat...

New parts? Internet vendors from Jensen to Ben's. eBay for hard to source parts. Local coop for really hard to source and otherwise cost prohibitive to ship small stuff or vintage. Sometimes Craigslist, too, for new take-offs or donor bikes.

jeebus, now that's a find!

I have had great luck finding parts at Bike Reyclery.

if you are in a hurry, and want close to the best price, Chainreaction Cycles is the first place to look at. Their prices may not be the lowest, but they have the fastest shipping of any UK shop, based on my personal experience.

I just received a very lightly used 7800 crankset that cost me $66.

whatever you do.. don't order from King Barcelona in Spain. some of their stuff is really great price. i ordered my Rotor QXL chainring back on 4/18/16 and tracking number shows it's still in their freaking post office preparing for shipment. i've learned my lesson.. was trying to save $15.. and now i'm stuck in a pickle fighting it out through paypal with that company.

Merlin, Wiggle, PBK, Ribbles.. i get my stuff in 2wks or less.

UPDATE: i finally got my chainring delivered as of yesterday. so roughly 1 month of wait. never again.

I've had good experiences ordering from:

Jenson: Jenson USA - Online mountain & road bike parts, clothing and accessories shop | Jenson USA
Universal Cycles: https://www.universalcycles.com/
Modern Bike: Bicycle Parts and Accessories - Modern Bike
Pricepoint: Your Local Mountain Bike, Road Bike, & Bike Parts Superstore
Bike Tires Direct: https://www.biketiresdirect.com/

I just paid LBS prices for a new cassette and chain to go along with the tuneup. After the 10% discount, it was $82 for an Ultegra 6700 11-28 cassette and $40 for an Ultegra CN-6600 10 speed chain. So quite a bit more than online. But since I was paying for a tuneup and had bought the bike there, they charged $0 extra labor to install. No complaints from me!

I have an affinity for Wiggle.