Over here:
myreadingmanga.info/attack-on-titan/
Keep on search you'll found it there's a ton of it!
Also google for key and cravat livejournal you need to become their member before you can download they works but please do not reupload anywhere! ٩(๑❛ᴗ❛๑)۶
ALL RiEren doujinshis by Rock 'n Dolless have Levi as the seme =w=b They're incredibly beautiful too =D
Does anyone know any Eren X Levi doujins where Levi is the seme? I can't find anything T-T