They blow up the world and everything has to be on round bases now. They bring back that world and everything (supposedly) goes back on square bases. They introduce a new edition and your models now need a larger base. They release new models you want to use for your existing army which is on obsolete bases.
There are lots of reasons for getting bases. Some you can still get via official routes. eBay can supply a lot. But maybe it would be nice if we could collect some links to reliable online stores which sell good quality bases. Maybe sort by type (classic plain black plastic, MDF, acrylic, textured resin etc) and/or region where it is available?
This is a wiki post. Add sources at your leisure.
Green Stuff World: Classic square bases in popular sizes, with slot for a 5x1mm neodymium magnet. Also round plastic, acrylic and MDF bases. Ships from Spain.
Kromlech / Bits of War: Classic square bases in popular sizes. Also textured resin bases. Ships from Poland.
Fluid 3D Workshop: Acrylic and MDF bases. Ships from the UK.
Bases & More (formerly Base-X-of-War): Textured resin bases in various themes, shapes and sizes. Ships from Germany.