They need to be made to fit your tractor to ensure everything is at the proper angles and has the proper amount of adjustment. There are some "standard fit" types that will work on many popular tractors fine. Most of these provide a more limited adjustment than the manual adjustment the tractor shipped with, or they'll be missing a bend that allows proper alignment on the tractor. It will be up to you to notice any small issues and decide how best to correct them.
Brian takes all the guesswork and wondering out of the equation and provides you exactly what you need for a thought-free installation on whatever you own. You won't need to modify anything, guaranteed.
I think he has a life and a day job. I'm pretty sure he's simply providing the service of his knowledge and spare time. Perhaps if he had enough business to pay the bills, turn-around times would improve. Perhaps for a reasonable fee, you can have him 'rush' your order. Perhaps the 3 month wait you had on the top link is not typical and you'll receive the side link much faster. Have you contacted him and inquired about an ETA?
Even larger companies can have wait times while they prepare your product for shipment. I recently had to wait more than a week for everything attachments to build a grapple to ship to me. I don't mind. When a company is building a product for a customer with a name, I think more care is taken than if it is being made to put on a shelf to wait for a customer to come around.