Really, there's no need to mail them about it. Just deal with it when you get there - it may not even be a problem.
Then, when you get there, IF there is none, ask the staff. Either it's missing and they forgot to put some there, and they'll refill it. Or, they'll be used to non-Bangladeshis asking about this, and will offer you some suggestions. Even if they find it a bit odd to ask, they work in tourism and realise that people are different and may not be used to their ways. Be polite and semi-apologetic about it, and it's pretty hard to offend.
And if the grocery store sells it, then under no circumstance would it be rude to ask them for some.
However, another suggestion is to embrace the local customs, and do what they do. Often there'll be a supply of water - a container or tap. Take some, use that to wipe with, and then wash your hands afterwards. It works for millions of people, and some argue that it's even better than toilet paper.
Odds are if they have a western-style toilet, they'll have paper. If, however, it's a squat toilet, we have a useful page on how to use those, if you're interested.