I enjoy using 3d terrain with miniatures games. Do you have any recommendations for terrain/buildings that would work well with Imperial Assault?

I haven't seen any full replacements, but you can get pieces that replace the tokens and sit in the impassable terrain to give sort of a faux 3d look to the board.

http://combatzone-scenery.co.uk/epages/e25b18fb-c32b-422a-8493-a8baeb0f1a17.mobile/en_GB/?ObjectPath=/Shops/e25b18fb-c32b-422a-8493-a8baeb0f1a17/Categories/Land/Star_Wars_Imperial_Assault

I got my stuff here, looks really good:

https://www.etsy.com/shop/GamerPlastic?ref=l2-shopheader-name

Paper terrain works well too and can be scaled on your printer. A quick Google search should give a ton of options.