I appreciate this will be a somewhat unpopular view, but...
There are loads of 'professional' sites out there that have good quality content. The Links section of this site links to a good number of sites, most of which update at least one per week, sometimes two or three times.
I think that good work should be rewarded and if you're into this kind of thing (and thus want it to be made), then you should encourage there to be a market for it by signing up (paying!) for a site which produces the kind of content you want. Everyone here is over 18, so practically all of us can get/use a credit card to subscribe if they really wanted to. If no one is willing to pay for this stuff then good quality, well produced content of this kind will be by far the exception to the norm. If you're buying diapers and aren't incontinent, then clearly you're already wiling to pay for this hobby/fetish/kink/personality-quirk/whatever, so the argument that you wouldn't spend money on this stuff shouldn't/doesn't fly.
Practically all the worth-looking-at content that appears on the sites mentioned above is either previews / ads from pay-site produced content or has been stolen from those sites and re-posted without permission. Of course there are exceptions to this (e.g. furry/anime stuff tends to be more "open" and gets posted to places like DeviantArt and "amateur but realistic" stuff tends to get posted to reddit/imgur and Tumblr every now and again), but generally its pretty low quality, low quantity stuff.
Even if you don't keep a subscription running all the time (e.g. you only subscribe for a few months a year) and/or you change between sites say, every 6 months or so, you'll generally get much better quality content and encourage the market to continue to produce this kind of material by virtue of the fact you create a proper market for it.
If you're looking for good ABDL
porn pics/vids/stories/etc., how about supporting those people who are trying to make a living by creating it?