As you completed MBA in Marketing and now want to do diploma course
Here I suggest some diploma courses that you can do.

Some courses after MBA in Marketing :

Post Graduate Diploma in Marketing Management
Diploma in Retail Management

Certificate Programme in Sales and Marketing

Diploma in Marketing Management

Post Graduate Diploma in Advertising, Marketing and Sales Management

Diploma in Sales and Marketing Management

Post graduate Diploma in International Marketing
Post Graduate Diploma in Distribution & Marketing

Some colleges that provide diploma Courses After MBA Marketing:

Aastha Institute of Management and Technology
Bhavan's Rajendra Prasad Institute of Communication and Management , Unit No. 2, Bhavana's Campus, Munshi Nagar, P.O. Box No. 57373 Andheri (West)
Mumbai, Maharashtra

Indian Institute of Commerce and Trade (IICT)
Sapt-Rishi, 5/28 Vikas Nagar
Lucknow, Uttar Pradesh

Jagran Institute of Management and Mass Communication (JIMMC)
F-33, Sector 6,
Noida, Uttar Pradesh

Mahatma Jyoti Rao Phoole Post Graduate Mahila Mahavidyalaya
Ram Nagar Ext., New Sanganer Road, Sodala,
Jaipur, Rajasthan

K.C. College of Management Studies
K.C. College Building, 1st Floor, D.W. Road, Churchgate,
Mumbai, Maharashtra

Sachchidanand Sinha College
Aurangabad, Bihar