What Rick suggests is exactly what I do, since I can't find those good old thumbpicks anymore. Golden Gates are more expensive but they're tough and well made. The one other thing I have to do is some careful trimming with a stout pair of scissors or tin snips so the thumb loop doesn't get caught in the strings. After the trimming I use fine sandpaper or emmory cloth to polish the rough edges.

The other thing I miss from the 60's are a certain type of plastic fingerpick that worked well for guitar. I haven't found a suitable substitute for those yet, though I get by with Dunlop metals like I use on the banjo.