There are widespread shortages of electronics parts at the moment, especially passives. Quoted factory lead times are 40 weeks or thereabouts, and since the industry is capacity-limited, it isn't clear that the situation is going to get better any time soon, so everybody's starting to panic. Given all this churn I've been spending an unconscionable amount of time lately finding suitable replacements for out-of-stock parts.
High value ceramic caps are the worst--their capacitance drops by at least 60% and at worst 95% at rated voltage, so finding an adequate substitute involves a lot more than the package, value and voltage rating. Most of their data sheets are useless, which is frustrating. However, all is not lost: most makers have websites where you can look at the C(V) curves.
Here's an alphabetical list. Many of these links can also be used for resistors, inductors, and other component types as well.
AVX SpiCat (This one is super clunky--every time you select something it displays a throbber for 5-10 seconds. It does give you soakage models though, which is nice if they're vaguely accurate.)
Cornell Dubilier has a lifetime vs temperature-calculator
Kemet KSIM (About the best; slow on some browsers)
CAD for Kyocera capacitors
Murata SimSurfing (Honourable mention):
Panasonic has downloadable selection tools that don't run under WINE, so I don't know if they're any good.
Taiyo Yuden TY Compas (Honourable mention):
Yageo: selection tools
Samsung has some good searchable datasheets that you can get to from e.g. Digikey's product page, but a lot of their products are the pits, e.g. this one, whose capacitance falls off by over 90% at rated voltage. Note that you want the characteristics link and not the datasheet link.
Others don't seem to, e.g. Johanson, Vishay, and most of the smaller Chinese outfits. Sure would be nice if everybody had decent datasheets like Samsung's better ones.