It would be great if it worked like that but I doubt it. Maybe if you were working with a middleman it would be different, e.g. some guy loses his hookup, he would probably work to get a new one or else he loses out on whatever he gets from being a runner.
If it's a proper dealer, then I'd say it just depends on the person. If you were genuinely friends (not just a business relationship) then I'd say your friend wouldn't mind trying to get you sorted from someone else. If not, then it just honestly depends on their personality. They have no duty to help you find a new hook up, both buyer and seller take risks and both help eachother out (the dealer needs money, the buyer needs drugs) but if the seller decides not to sell anymore then that should be the end of it. Find someone new.
Maybe if they like you a lot you could ask them nicely if they can get you sorted from someone else (this would depend on how likable and trustworthy you are in the dealers eyes but it's unlikely to happen), but if not you're just going to have to leave it at that and find someone new.
The dealer should never have to worry about that. As someone getting supplied with illegal substances regularly at a home address, you need to be a bit trustworthy. If the dealer trusts you enough to do this he shouldn't ever have to worry about anyone ratting him out either before or after he stops selling. If that worry is present, then he/she has made a mistake.
If the dealer for *some weird reason* decided that you might tell on him after he stops selling, giving you on to someone else isn't the best thing to do. If you have the potential to rat him out, then you would have the potential to rat out the new dealer - making you a very undesirable customer. Even after getting sent to a new dealer, you still can snitch on the old guy, so only new problems have been created.