ME_PROCESS_PO_CUST is the main MM module
However, the following exits also exist.

Transaction: Select the component from SMOD and see if it meets your requirements.

AMPL0001 User subscreen for additional data in MPL
LMEKO002 Priced extended communication structure KOMP
LMELA002 Adopting lot number from shipping notification at goods receipt posting
LMELA010 Inbound Delivery Notification: Transfer Item Data from IDoc
LMEQR001 Supplier Exit User Exit
LMEXF001 Conditions in Purchasing Document without Invoice Receipt
LWBON001 Communication structure MCKONA (transaction volume rebate) expansion
LWBON003 Change settlement data before registering settlement slip
Customer-specific source determination for LWSUS001 AP Retail
M06B0001 Function determination for purchase requisition approval
M06B0003 Number range and document number
M06B0004 Number range and document number
M06B0005 Communication Structure Change: Requisition Header Level Approval
M06E0004 Changed communication structure of approval purchase order
M06E0005 Function determination for purchase order approval
ME590001 Grouping purchase requisitions for purchase order split with E59
MEETA001 Scheduling plan type definition (backlog, immediate request, preview)
MEFLD004 Goods receipt (only for purchase order reference) Determining earliest delivery date for confirmation
MELAB001 General Demand Forecast Delivery Schedule: Transfer Plan Introduction
MEQUERY1 Expansion: Document Summary ME21N / ME51N
MEREQ001 Customer in-house data in purchase requisition
MEVME001 E Initial quantity calculation and over / under delivery tolerance
MM06E001 DI User exit for inbound / outbound purchasing documents
MM06E003 Number range and document number
MM06E004 Import data screen management in purchase requisition
MM06E005 Customer-defined field in purchasing document
MM06E007 Document change for purchase requisition converted to purchase order
MM06E008 Monitoring basic contract target amount when ordering release
MM06E009 Text Registered “Text for Indicator
MM06E010 Field selection for vendor address
MM06E011 ctivate PReq Block
MM06L001 Exit for vendor evaluation settings
MMAL0001 LE Supplier List Distribution: Outbound Processing
MMAL0002 LE Supplier List Distribution: Inbound Processing
MMAL0003 LE Purchasing info distribution: Outbound processing
MMAL0004 LE Purchasing info distribution: Outbound processing
MMDA0001 Delivery address default value
MMFAB001 User exit for release order generation
MRFLB001 Manage Contract Release Order Item