public class OpportunityShipToSelectExtension {
    private final ApexPages.standardController controller;
    private final Opportunity obj;

    public OpportunityShipToSelectExtension(ApexPages.StandardController stdController) {
        this.controller = stdController;
        this.obj = (opportunity)stdController.getRecord();
    }

    public SelectOption[] getShipToOptions() {
        SelectOption[] ShipTo = new SelectOption[]{};
        ShipTo.add(new SelectOption('','--None--'));
        for (ERP_Address__c A : [select Id, Name , Address_Line_1__c, Address_Line_2__c, Address_City__c, Address_State__c FROM ERP_Address__c where ERP_Address__c.Bill_To__r.SFDC_Account__r.ID = : ApexPages.currentPage().getParameters().get('AccountId') ORDER BY Address_Line_1__c ASC NULLS FIRST LIMIT 500]) {
            ShipTo.add(new SelectOption(A.id, A.name + ' (' + A.Address_Line_1__c + ', ' + A.Address_Line_2__c + ', ' + A.Address_City__c + ',' + A.Address_State__c + ')'));
        }
        return ShipTo;
    }
}

<apex:page standardController="Opportunity" extensions="OpportunityShipToSelectExtension" showHeader="false" >
<apex:form id="ShipToForm">
<apex:pageBlock title="" mode="edit">
<apex:pageBlockButtons >
<apex:commandButton action="{!save}" value="Save"/>
</apex:pageBlockButtons>
<apex:pageBlockSection title="Select Ship To from Available Addresses" columns="1">
<apex:pageBlockSectionItem >
<apex:outputLabel value="{!$ObjectType.opportunity.fields.Ship_To__c.label}" for="pLabel"/>
<apex:outputPanel styleClass="requiredInput" layout="block">
<apex:outputPanel styleClass="requiredBlock" layout="block"/>
<apex:actionRegion >
<apex:selectList id="ShipToLookupPicklist" value="{!opportunity.Ship_To__c }" size="1" rendered="true">
<apex:selectOptions value="{!ShipToOptions}"/>
</apex:selectList>
</apex:actionRegion>
</apex:outputPanel>
</apex:pageBlockSectionItem>
</apex:pageBlockSection>
</apex:pageBlock>
</apex:form>
</apex:page>