Which table/record I need to source to create a select field in a suitelet which has Shipping Method> The field I am trying to replicate is having internal id on transaction record for the field is "shipmethod".

I think you're looking for '-192'. For example:

form.addField({
    id: 'custpage_test',
    label: 'Shipping Method',
    type: ui.FieldType.SELECT,
    source: '-192'
});

To find the value for these internal lists, I go to the create custom field page and set up a List/Record > Shipping Method field type, then look at the value in the List/Record field. In this case, I'm using the Advanced Field Help plugin, but it shows that Shipping Method has a value of '-192'.

oh I didn't know we could source with the numeric value, didn't notice it earlier. Thank You.