We have multiple suppliers with multiple warehouse addresses and I want there to be a field on the PO which can provide a drop-down for someone to select the address it should be.. I was thinking of adding all these addresses on the supplier record under the address tab however not sure how to source this or is this the best place to put it.. are there any alternative methods I could use?

You don’t have all your warehouses as locations in NetSuite? The shipping address on the PO is from the location you assign

So these are all outsourced warehouses.. i.e. we have the warehouse called A with multiple pick up points, trying to get this to be selectable on the PO so it's not broken down on the location as warehouse A pickup point 1, warehouse A pickup point 2 etc..

I see You can make a custom record to hold this information. Then add it as a list/record field on the PO. You could also use a WF to source the full address text from the custom record into the text area ship to field as well