I'm running out of the tubing (keeps the trailer hook from coming loose) that comes in the pack with trailer hooks, but I have plenty of hooks, so I don't want to just buy another pack. What's a good alternative for trailer hook keepers?

surgical tubing.

any med. supply store.

Or you can cut up a piece of plastic, such as a lid that might come with a can of coffee. I cut little triangular pieces so there's something to grab onto when I want to remove it.

Get a 3 ring binder and paper hole punch. Remove plastic outer and punch away.

Garnet

All of the above are good ideas and work, but all i use is a piece of soft plastic worm ,lizard,crawl or whatever i have handy that has gotten torn that day.

I use rubber screen spline tubing, that is used to hold screen wire in the frames of your house windows.

Cut a wide rubber band into small pieces.  This works good.  You can also cut the rubber band into a longer piece and double it.

The plastic hook gard that comes on some new spinner baits work good.  They're hard to get on sometimes but work.  Just cut to the size you want.

Really any soft rubber will work.  You can buy rubber or plastic tubing at most hardware stores that will work.  Using pieces of old rubber bands it the cheapest and easy.