You can't get Hong Kong visa on the entry ports, but you can apply for a Macau visa on arrival at the entry ports of Macau.

If you just enter Mainland China once, you need a single-entry China visa, you also need to apply for Hong Kong visa, Macau visa on arrival.

as for the Hong Kong visa application:
Embassy of the People's Republic of China in Bangladesh may issue single/double journey transit or visit visas valid for 3 months for a stay up to 7 days for transit or up to 14 days for visit to the following categories of Bangladesh citizens including their accompanying spouses or children:
1. Government officials holding ordinary passports;
2. Prominent persons in the local society or business communities;
3. Employees of well-established or reputable organizations and companies including employees of multi-national companies, banks, financial and insurance entities, airlines/shipping companies and foreign organizations in Bangladesh;
4. Persons invited by the HKSAR Government or reputable organizations in HKSAR (e.g. the Hong Kong Trade Development Council).

IV. Applicants not included in the list of above applicants may mail their applications for visas directly to the Immigration Department of the HKSAR.

Contact Information:
Tel: 00852-28246111
Fax: 00852-28777711
Website: http://www.immd.gov.hk
E-mail: immd.gov.hk|enquiry
Mail Address: Immigration Department Immigration Tower
7 Gloucester Road Wan Chai Hong Kong

it takes about 4 weeks for Hong Kong visa application if you send application by post to the immigration office.