Soft: https://www.amazon.com/gp/product/B07JHZBVQ9/ref=ppx_yo_dt_b_asin_title_o02_s01?ie=UTF8&psc=1

Hard: https://www.amazon.com/gp/product/B003C23PFU/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

EDIT: The BCW hard sleeve and soft Ultra Pro sleeves seem to NOT fit together. Go with Ultra Pro instead if you intend to double sleeve:

https://www.amazon.com/gp/aw/d/B000VVCPUG?psc=1&ref=ppx_pop_mob_b_pd_title

I've gotten both those and the Ultra Pro hard ones and BCW seems to be better. Fewer scratches all over them upon delivery and they are cut better. Some of the Ultra Pro ones stick together and need to be physically broken apart. The seams on the plastic on the Ultra Pro ones are also lower quality such that there are dangling plastic "strings" and lots of air bubbles.

Titan Size Soft (not perfect but best fit. Does not perfectly fit inside the hard one, it will hang out of the top by a few millimeters): https://www.amazon.com/gp/product/B00TJQZ31W/ref=ppx_yo_dt_b_asin_title_o02_s02?ie=UTF8&psc=1

Titan Size Hard (not perfect but best fit. Putting a soft inside does hang out of the top a little bit): https://www.amazon.com/gp/product/B00V0EYDYY/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

As for combiners, unfolded they are the same size as a regular character so they will fit with these the same way. I know lots of people plan on leaving them fully unfolded inside a sleeve and then using a marker of some kind to show their mode. My tip for that is that a character card is exactly the size of two battle cards. So any spare battle card can be used to cover up one side or the other.