I just responded to a similar question in another thread but wanted to share here too - over the years I have learned that being honest and direct seems to work the best. I try to express in the kindest way possible why I don't want to make anymore.
"I am so flattered that you liked my gift so much!! Unfortunately my time is limited so I cannot commit to making more. When I find the time I make them to give but I don't always have that time - but I will help you as best I can to make your own!" This is sort of my go to response.
If that doesn't do it I say, "I am so grateful to be appreciated for my sewing, but I limit my personal sewing to when I have the time to do it otherwise it's just no fun for me anymore. I do know a lady who makes these for hire..." or "...there's this website called Etsy, maybe someone would custom make one for you/you're friend/you're coworker/you're daughter?"