Adult diapers are widely available, if not at the pharmacy then at shops that cater to the elderly and/or infirmed. Many people who are incontinent or confined to wheelchairs use them regularly to prevent embarrassing accidents. They're not perfect, you might have a bit of an odor, but you won't leave a giant puddle. Things might get a bit unpleasant if you are required to enter secondary inspection at a port of entry, but if you're not embarrassed and are not smuggling drugs in your diaper, it's their problem I guess.
The US military has some systems that are compatible with both varieties of factory pilot plumbing, but they're $2,000 each, so not likely to be practical for a typical Greyhound passenger.
There are also accessories on some small planes that can be used by pilots who have their own appropriate matching plumbing factory-installed. They are called "pilot relief tubes" and just conduct the urine to the great outdoors where it can be enjoyed by woodland creatures and whatever else is under your flight path.
Aside from "personal" devices, aircraft supply houses do have some other options, which would probably not be suitable for use in public. They're basically a disposable bag with some gel. Trying to use water bottles and airsickness bags has been done, but the results reportedly can be sub-optimal.