How would one go about forging such relationships
Firstly you should already be selling in amazon and making money.It is very easy to convince them to allow you to sell their products when you are already doing well. Maybe 5k a month plus revenue.
I would just check out what wholesalers are selling. There are plenty of wholesale lists available online, just tailor the search to your home town.Make sure to check if the product is profitable using Jungle scout. It will give you a lot of info including the number of sellers on amazon and the number of units of that product sold per month.
Example a product shows up as selling 3500 times a month. There might be 11 sellers of that product. The only sellers that will really make money are the ones that are doing FBA(fulfilment by amazon where basically you send your stock into amazon and amazon then boxes it and sends it to a customer when they buy it)and the ones with a selling price within 2% of the buy box. Say there are 4 sellers that match this description, they will share at least 3200/3500 sales. This will mean you will get roughly 800 sales(3200/4) so if you were to make say £4 per item then that you will make £3200 a month for that item then you need to just keep finding many many items that will make you good money.
I would also then create a website for myself and a business card when i start doing very well for when I would go to different trade shows and give them out to the people you are speaking to. They get so many people going to them that they need to have something to remind them of you which is what your card does. I haven't got to the level where i need a card or a website.
I don't think i have explained it well but i think you will get the jist of it.