Lots use Progressive brand progressive (Surprise, Surprise) fork springs with 7 w oil up from 5w stock.
04-12 Triumph Rocket III Progressive Suspension Fork Springs 11-1529
Progressive Suspension - 11-1529 - Fork Springs Kit Triumph Rocket III | eBay
I put Progressive's in mine with 10w oil but I am a 350lb lump. I also have their top of the line Progressive brand 444 model rear shocks.
Whatever you decide on, do some Google AND eBay searches to find best prices WITH postage to your location. Do NOT ignore ebay listings from other countries or posted in other languages or currencies as even after Paypal currency transfer fees and postage these can often be cheaper with exactly the same Paypal and eBay protections.
You can also get Wilbers made but branded Promoto front fork springs:
457742 PROMOTO Fork springs TRIUMPH Rocket III Touring 23 XC 07 | eBay
Promoto Fork springs Triumph Rocket III Built from 04 | eBay
as well as Wilbers branded ones:
Kit Springs fork Wilbers + 2x L Oil WP Triumph Rocket III | eBay
TRIUMPH ROCKET III Roadster Fork Springs stock height | eBay
Do your fork and dust seals at the same time:
Triumph Rocket III 2300 2004 - 2012 Fork Oil & Dust Seal Kit - With Oil
or just fork seals for less:
Triumph Rocket III 2300 2004 - 2012 Showe 3 Fork Oil Seal Kit | eBay
and lower range end rear Wilbers for slightly more than Progressive rears but Wilbers have a large range of higher end rear shocks as well. Wilbers are great quality but you do pay for it.
For most Rocketers Progressive 444 or Hagon / YSS or Shock Factory quality/performance is a good compromise.
Lower grade Progressives would be better than stock as you can specify spring rate.