There are four cushion shapes commonly used today, with the majority of tables using a profile called K-66. Although I can source any brand of cushions you request, I recommend Mercury Ballistic Speed cushions, which have exhibited excellent consistency and long life. There are many cheaper, inferior brands that just do not last.
Like cheaper cushions, there are low-quality adhesives that may fail and let the cushions separate from the rail over time. I only use Barge Cement when changing cushions, as I believe it is the finest product available.
Whatever style pocket your billiard table has, they take a beating and may eventually crack and fail. For most tables from the past 50+ years, they can simply be replaced. However, it is important that Antique Pool Tables keep their original pocket irons, as they were individually fitted to the table as it was manufactured, so only the leather should be replaced – if needed to bring the table back up to snuff.