What belt with this dress?

[UntilTheCowsComeHome]

Just got this dress in navy.

I'm a lumpy, bumpy size 14 and I hoped it would skim over my flab curves but it looks really shapeless.
So I tried a skinny belt around the thinnest part of my waist and it looked heaps better.

The only belt I own is tatty so I need to buy a new one. I was wondering if a thicker belt might look even better or do I stick with the skinny belt idea? If so do I get a really understated one or one with a thicker buckle detail?

I'm crap at accessorising and I haven't worn a dress since my wedding day, help!

[UntilTheCowsComeHome]

Also where can I shop that would stock a good variety of cheap belts? I can only think of primark and I can't get there any time soon.