Not so much Darts, but Wilts & Dorset is good 'stepper' territory. Poole depot retains a good handful of Olympians, mainly ex-Southern Vetis and including a good few with ECW bodies, I believe they can be found in the New Forest too. They are spread over the main W&D fleet and also the associated Damory Coaches outfit, and whilst they're mainly used on irregular runs like schools routes etc, I'm pretty sure you can get some mileage in down that way, and certainly some pics. Always plenty lined up outisde Poole depot, you could get in touch if you wanted specific info.
Slightly further afield you could have a look at First Hants & Dorset, there are a few step Darts around the Portsmouth area, though getting rare now. Hilsea certainly have at least one, it's a Pointer on an N plate and originally with Southampton Citybus I believe, transferred when they merged it all. Hilsea also still has minibuses if that floats your boat at all, Merc Varios with Plaxton Beaver 2 and Marshall Master bodies. Hoeford (Fareham) did have a batch of pretty rare Darts with UVG bodywork, but I have a feeling they've all been scrapped or transferred away now.
If it's mainly Darts you want, try Emsworth & District. They have a few ex-Stagecoach J-GCD examples with Alexander Dash bodies, and at least one Pointer. They also have some rarities, including G516 VYE, an ex-LT Airbus Dart with Carlyle Dartline body, a Titan and some very obscure Reeve Burgess Harrier-bodied Leyland Swifts (like a Dart). As well as their main depot, they also have a compound known as '
Steptoe's Yard', which contains the remains of most of everything they've ever operated, quite interesting. The chap who runs it is a bit of an enthusiast, could be worth an email.
http://www.flickr.com/photos/southdown/3138395106/in/photostream/
Carrying on up the coast, Stagecoach at Chichester still run the standard Volvo Olympian/Alexander RL in the fleet.