Here are some app rips I pulled yesterday from an Amazon Fire TV 3 or found while searching for your Shield, Mi Box, etc. These weren't in the Play Store.
I found that the Fox Now app from the Play Store was limited. I first asked for a rip from a Fire TV but then bought one to do myself. I figured I would try to give back some to the community.
SyFy 3.1.218 (Tested, icon isn't full length) https://www.dropbox.com/s/bt843vpabwdu043/SYFY_3.1.218.apk?dl=0
USA 3.1.218 (Tested, icon isn't full length) https://www.dropbox.com/s/aoglcsv9epaxhtl/USA_3.1.218.apk?dl=0
Firefox 2.0.1 (Tested, works) https://www.dropbox.com/s/nim48qjrzytb7n1/Firefox_2.0.1.apk?dl=0
AMC 2.0.69 (Tested, works) https://www.dropbox.com/s/7yzytuama6l7mm1/AMC_2.0.69.apk?dl=0
Globo Play 1.12.0 (Tested, works) - Brazilian TV https://www.dropbox.com/s/jcczqneovjwu136/Globotv_1.12.0-10074_minAPI21%28arm64-v8a%2Carmeabi%2Carmeabi-v7a%2Cmips%2Cmips64%2Cx86%2Cx86_64%29%28nodpi%29.apk?dl=0
Directv Now Beta 2.0.10.05169 (Tested, works) - Must also install Amazon Appstore and add Directv Now Beta to account (See Below)
https://www.dropbox.com/s/19c0c8fdi1j9hrp/DirecTVNow_beta_2.0.10.05169.apk?dl=0
Login with ATT creds not Directv Now.
Directv Now - Must also install Amazon Appstore and add Directv Now to account (See Below)
https://www.dropbox.com/s/pfkhuani9pjabuo/DirecTVNow_amazon_1.0.22.4821.apk?dl=0
Amazon Appstore 31.31.1.0.200418 (Tested, works) - Login with keyboard/mouse if necessary https://www.dropbox.com/s/mnbubjztr4gpy9r/Amazon%20Appstore_release-31.31.1.0.200418.0_800236410.apk?dl=0~~
Amazon Video 4.2.43-nvidia (Tested, works for those needing Amazon Video on a Mi Box) https://www.dropbox.com/s/sfc043jaacryg2h/Amazon%20Video_4.2.43-nvidia-armv7a.apk?dl=0
TruTV 1.0.9 (Tested, fail on Mi Box and Nvidia Shield for me) https://www.dropbox.com/s/sa0fxbhdooylmua/TruTV_1.0.9.apk?dl=0
History 1.6.3 (Tested, fail on Mi Box and Nvidia Shield for me) https://www.dropbox.com/s/n1uf6h622ziaqoi/HISTORY_1.6.3.apk?dl=0
AE (Tested, fail on Mi Box and Nvidia Shield for me) https://www.dropbox.com/s/i7qunrz051ncdjj/AE.apk?dl=0
Fox Now 3.4.1 (Tested, fail on Nvidia Shield for me) - App in Google Play Store works but outdated, no longer supported per developer. https://www.dropbox.com/s/tzu01ho5wycswqk/Fox_3.4.1.apk?dl=0
ADDED
Discovery Go 1.3.4 https://www.dropbox.com/s/2wzkyl060jsm5ti/Discovery%20GO_1.3.4.apk?dl=0
Discovery Life Go 1.3.4 https://www.dropbox.com/s/tr4ixys55jcdhkz/Discovery%20Life_1.3.4.apk?dl=0
Animal Planet Go 1.3.4 https://www.dropbox.com/s/amagn1si932zjzq/Animal%20Planet_1.3.6.apk?dl=0
Sci Go 1.3.4 https://www.dropbox.com/s/8drjbn987fvl8zl/Science%20GO_1.3.4.apk?dl=0
TLC Go 1.3.4 https://www.dropbox.com/s/6u4hsa7qnn4yw9z/TLC%20GO_1.3.4.apk?dl=0
Destination America Go 1.3.4 https://www.dropbox.com/s/gjk2gypsnr28dbi/Destination%20America_1.3.4.apk?dl=0
Velocity Go 1.3.4 https://www.dropbox.com/s/dkbuqcs3vay1nx6/Velocity%20GO_1.3.4.apk?dl=0
CW 1.2 https://www.dropbox.com/s/qsw5v7hrs5kfmrz/The%20CW_1.2.apk?dl=0
CW Seed 1.0.1 https://www.dropbox.com/s/yfioqxzdyfu9q2f/CW%20Seed_1.0.1.apk?dl=0
Fox Sports Go 3.8.0 https://www.dropbox.com/s/wr9lt5hmgt9kvx2/FSGO_3.8.0.apk?dl=0
NBC Sports 3.2 https://www.dropbox.com/s/cl0gujsl8bxsp4g/NBC%20Sports_3.2.apk?dl=0
Nick 1.1.2 https://www.dropbox.com/s/z9eqn1kdwvl90jg/Nick_1.1.2.apk?dl=0
Nick Jr 1.0.2 https://www.dropbox.com/s/vsrbe99mgwvrfnr/NickJr_1.0.2.apk?dl=0
Amazon Music 2.9.200353 https://www.dropbox.com/s/3uu9hadw737aehn/Amazon%20Music_2.9.200353.0-prod_294451010.apk?dl=0
Disney Channel 3.25.0.472 https://www.dropbox.com/s/8yb9gj1au1u6qvk/Disney%20Channel_3.25.0.472.apk?dl=0
Netflix 5.1.1 https://www.dropbox.com/s/3km2pjckksspure/Netflix_5.1.1.apk?dl=0
ADDED 4/17/2018
Hulu 4.2.6.555 https://www.dropbox.com/s/4e8z628utqx993i/Hulu_4.2.6.555.apk?dl=0
Boomerang 1.10 https://www.dropbox.com/s/ispnocpse3kxp84/Boomerang_1.10.apk?dl=0
Tennis Channel 7.0 https://www.dropbox.com/s/ktfb6up989saneq/TennisChannel_7.0.apk?dl=0
IFC 2.0.31 https://www.dropbox.com/s/lteilco50n5sy8s/IFC_2.0.31.apk?dl=0
ADDED 4/19/2018
DramaFever 1.00 https://www.dropbox.com/s/1kgunrq46yx21r0/DramaFever_1.00.apk?dl=0
Viki 2.0.3 https://www.dropbox.com/s/vka7wz7ua5bmvqi/Viki_2.0.3.apk?dl=0
Bob Ross Joy of Painting 1.00 https://www.dropbox.com/s/zo5ag3jujalmgrs/BobRossJoyofPainting.apk?dl=0
ID Go 1.3.6 https://www.dropbox.com/s/rjighcpjjvomyju/IDGO_1.3.6.apk?dl=0
Directv NOW Beta 2.0.10.06114 (Needs latest Amazon App Store Below) https://www.dropbox.com/s/1dtyfwfp73uaxxl/DIRECTVNOWBeta_2.0.10.06114.apk?dl=0
Amazon App Store https://www.dropbox.com/s/c3p26mjvvzkfdzb/Amazon_App.apk?dl=0
These apps haven't been stripped of Amazon requirements. For many, you have to find the app you sideloaded in the Amazon app store that you installed from above that you logged into and add the app you are sideloading. Then your Amazon account shows that you "own" it allowing you to use it. That's what I originally did for the Directv Now apps I found and sideloaded prior to getting a Fire TV to pull the APKs myself.
If you have a hard time getting around the Amazon purchase issue, lucky patcher can remove Amazon app store dependencies from an apk.
Works with ~50% off apps.