Someone asked about bicycle availability in Dhaka. Basically there are two kinds: the traditional ones (Phoenix is a popular brand) and the newer Mt Bikes from China. Although I have not tried either in recent years, what I hear is that the Chinese Mt Bikes, meant for kids or leisure use, are flimsier than the traditional ones.
If you plan to bicycle a lot in Bangladeshi countryside you will need a sturdy Mt Bike. Your best bet is to bring one from abroad. They can be boxed and checked in as airline luggage (may be excess baggage?) We brought our (somewhat older) bikes from the US in our shipping container. Also get some basic tools and spare spokes and tubes. (My Schwinn recently broke 5 (!) spokes and I had a heck of a time getting it repaired here.)
There is also the Bangkok option. Several expats I know picked up a Mt Bike when they were in Bangkok, and checked it as luggage. You can get some good ones there, including a brand called Trek which seems really nice.