Depends on your age/budget.
If you're looking for a 'classic' mother of the bride style outfit, Jacques Vert is always very good with a very wide range of sizes, and you can have a good browse online. This will set you back a couple of hundred quid though, but you will get a matching hat in the range. There are huge markdowns in their sale online and in store, too.
The big department stores like Debenhams and John Lewis both have very good sales online/in store with a wide range of brands, and a number of brands that cater to the ladies formalwear market.
The spring sales are starting/have just begun in most places, so this is a good time to go hunting around and see what's available and if there's anything you like.
If you're thinking of ordering from the big Chinese dropshipping sites (Shein etc) I would urge you to order in lots and lots of time as some items are really quite different to the promo shots. On the other hand they are also consequently drastically cheaper.
Good hunting!