23. Is there a ban on political parties engaging in commercial activities?

Source: 

Section 8, Parliamentary Elections Act, No. 1 of 1981 

8. Recognised political parties to comply with certain requirements.
(1) It shall be the duty of the Secretary of every recognised political party to submit to the Commission a copy of the Constitution of such party together with a list of the office bearers of such party and a copy of
the political programme of such party—
(a) in the case of a recognised political party referred to in subsection (2) of section 7, within three months from the date of coming into operation of this Act; and
(b) in the case of any other recognised political party, within three months from the date on which such party is entitled to be treated as a recognised political party under paragraph (a) of subsection (5) of section 7.
(2) Every recognised political party shall hold a general meeting once a year or as specified in the Constitution of the party. The procedure to be followed at such meeting shall be specified in such Constitution.
(3) Where a recognised political party amends its Constitution or changes its Office bearers, the Secretary of such party shall, before the expiry of a period of thirty days from the date of such amendment or change, inform the Commission in writing of such amendment or change.
(4) A copy of the annual statement of accounts of every recognised political party audited by a registered auditor shall be submitted to the Commission.
(5) Every recognised political party shall be entitled to State assistance as provided for in the relevant written laws applicable to the conduct of elections.
(6) —
(a) For the purpose of exercising the powers conferred on the Commission Under this Act, the Commission may by notice in writing require the Secretary of any political party to furnish such information specified in
the notice issued, pertaining to such political party or to produce any document as shall be specified therein, within such period as shall be specified in the notice.
(b) It shall be the duty of the Secretary of any political party who is required by notice to furnish any information under subsection (1), to comply with those requirements within the time specified in such notice,
unless such Secretary is precluded from divulging such information under the provisions of any written law.
(7) Any person shall, on payment of a prescribed fee to the Commission have the right to call for and refer to the Constitution of a political party and on payment of the prescribed fee shall have the right to obtain
a certified copy of such Constitution.
(8) Commission may issue from time to time a Code of Conduct to be observed by political parties and candidates during elections.
(9) The Commission may from time to time issue such general or special directions to the political parties, a person or body as may be necessary for the effective and efficient implementation of the provisions of
this section and section 7.
https://www.srilankalaw.lk/revised-statutes/alphabetical-list-of-statutes/862-parliamentary-elections-act.html 

The Parliamentary Elections Act, No. 1 of 1981 outlines the obligations of recognised political parties. However, there is no provision in this or any other law prohibiting political parties from engaging in commercial activities.