I second the Ann Summers recommendations, their fishnet hold-ups last forever, and are lovely and roomy (they come up high even on me, and I'm a tall size 16 with long legs).
Ann Summers underwear can be quite pricey but I get a lot of their stuff on eBay or in the sale. I think they do a lot of things up to size 22/24, but I find their sizing tends to be on the small size. You can get steel-boned corsets from eBay too, some at very reasonable prices. I like the underbust corsets, which nip in your waist but leave your boobs naked (and clients like them too!)
For me, Pretty Polly lace-top hold-ups are the absolute best, though. I am very heavy-handed, and quite clumsy, but these stockings last through anything and everything! And they're a really good fit.