One bit of advice i would add is that you should think about the labels you are sending music to and does your demo fit with that labels style - I get sent around 60 to 100 tunes / Promos / demos a week and only about 3 or 4 at the very most fit with what I like or do - Less is more, No point in sending a tune out to tons of labels when its only gonna fit on a few
Also when I know a label demo has been sent to other labels it makes me less likely to want it as its only been sent to me as a after thought - Someone sent me something the other day and said " this was for label x but they didnt want it" I just think " well so you are now trying to pass other labels rejects off on me then cheers"
Also well worth spending the time to get a selection of DJ's behind you first as well - Send them tunes and see if they play then (again focus on DJ's who play the style of tunes you make) that way you can be sure your tunes are at a quality worth sending out - I have a mental list of producers who I dont bother downloading cos they send me half arsed unfinished tunes every week - They could be making amazing music now BUT cos they sent me really bad shit in the first place I just avoid new stuff as I don't have the time to deal with 3 minute clips of half finished neuro funk
#Waffle
To answer your original question - Most Dj's have a email on their twitter BIO of where to send tunes, Labels have info on websites also worth checking on Facebook pages for contact info as well
http://36hertz.com/demos/