Here's a post that will guide you to download the proper version of addons for the 7.1.5 client.
Community -> Addons page:
https://firestorm-servers.com/us/addons/index
Be aware that you find addons for 7.0.3 also here.
Popular websites for downloading addons:
https://wow.curseforge.com/addons - you will find most of the addons here.
You can download the version for 7.1.5 easily: in the "Files" section you find the dropdown "Game Version:" on the right side, select 7.1.5 here.
In the list of files look for the most recent release version (the one that has the most downloads, generally)
Eg. DBM (DeadlyBossMods) - https://wow.curseforge.com/projects/deadly-boss-mods/files?filter-game-version=2020709689%3A6464
And. GTFO - https://wow.curseforge.com/projects/gtfo/files?filter-game-version=2020709689%3A6464
https://www.wowace.com/ - you will find ca. 50% of the addons here (those using the Ace libraries), and uses the same interface as curseforge.
Eg. Bazooka for 7.1.0 - https://www.wowace.com/projects/bazooka/files?filter-game-version=2020709689%3A628
https://mods.curse.com/addons/wow - you find ca. 99% of the addons here ( ElvUI being the 1% :-D ), but the download has a 3 sec delay, aminor annoyance. Twitch also downloads from here.
Eg. Bazooka for 7.1.0 - https://mods.curse.com/addons/wow/bazooka#t1:other-downloads
http://legion.bestwowaddons.com/legion-addons-7-1-5/
http://www.wowinterface.com/addons.php - on this site it's hard to find the version you need, because the game version is not communicated. it seems only developers use this site now.
Eg. GTFO - http://www.wowinterface.com/downloads/info17996-GTFO.html#other
If you don't find a 7.1 or 7.1.5 version of an addon, then enable "Load outdated addons" in the Addons window.
7.0.0 addons have a 90% chance to work.
6.0.0 addons 60%.
5.0.0. addons 30%, but development and debugging addons still work pretty well (these are not really updated).
Good luck making your dream UI ;-)