In answer to your questions,
1 Most shoe findings suppliers sell wholesale but if you search for boot repair you may find a boot repair shop that will sell them to you. You could also build a stacked leather heel base. That is what most high end cowboy boots have.
2 It is hard to change heel height with out messing up the balance of the boot a half inch is a lot.
3 You can buy a product at most shoe store called leather stretch, it is a liquid that you spray on or rub on, put on the boots and apply then wear them till they dry. could take a few applications, or you will need a boot stretcher, depending on boot you can go half a size to a full size in width.
If you don't have the tools or equipment you might be better to send them to someone to have work done. If you search boot repair there are many places that can do the work. hope this helps