Replacing windows; where to purchase?

[Ian Saingarm]

Hi there, on a cedar shingle sided house with a mix of old aluminum and wood framed windows, is it best to just measure and order from a place like home depot?  We have all the siding off, so it seems now is the time, just want to figure out the most cost effective way.  Any help is appreciated!

[Ian Saingarm]

@David Hildebrandt thank you!  I did not know about thumbtack.

@Tom Shallcross thank you for all the advice!

I removed one window, it was aluminum frame within a wood frame, if that makes sense.  One of the windows I need to do is apparently actually a true wood frame window; large picture style window.  I can't seem to find the exact size windows so I was going to do slightly smaller windows, then build up the sides with some framing, and then get a drywall guy to tidy up the insides.  We will be doing flashing and the surrounding insulation like you mention.  LMK if that sounds right?  Much appreciated!