I'm in a city with quite a few shops and the only place that has a maternity section is H&M, and even then it's very small.
Really you are probably gonna have to buy some online.
Don't use Shein though, they are awful.
For basics I just went for H&M online and Amazon, and a few bits from supermarkets (George online are quite good but I've never seen an actual maternity section in an Asda!)
I got a few nicer bits from Mamalicious, Seraphine and JoJo (in the sales!)
Vero Moda and Vertbaudet are also worth a look.
There's loads of summer sales on at the moment so it's a good time to get stuff from nicer shops for a bit less.
But yeah - the high street offer is pitiful - just not realistic to get everything IRL unfortunately. Even M&S don't really do maternity stuff anymore other than bras.
I did buy a couple of things IRL that were just sizing up in regular clothes - you don't need special maternity versions of everything. If it has an elasticated waist or a relaxed fit anyway, you will prob be OK with a size or two bigger!