Consider making your own candi syrups:
20-lb-of-sugar-and-a-jar-of-yeast-nutrient
Read the last few pages in that thread too.
I use high gravity beer wort instead of water and like the results much better, more complexity.
Cooking up to D-45 or even D-90 is very attainable, even for a novice with some practice, a good reliable and accurate thermometer (e.g, Thermapen) and a sense of the process. The higher, the more challenging, although a homemade "D-120" turned out delicious.
D-1, D-5 are very simple, quick, and easy; D-45 is good practice before moving onto D-90.
Now D-180 or higher are a true challenge, and takes quite a bit of time and practice with plenty of trial and error. I can get close to a D-180 (possibly even better tasting without it being that dark). D-240 is still out of reach here, and IMO not needed in a Belgian Dark Strong. Even D-180 can be too acrid.
Regular white cane table sugar works fine. You could use any raw sugars if you want. The impurities can add more character and depth during the cooking process.
Do not use any "brown" sugars, as molasses needs to be avoided here (and in beer in general).
Here's a slightly different approach by
RyanBrews.
More importantly:
Belgian Dark Strongs need lots of good healthy yeast (large starters), oxygenation when pitching, and patience, from controlled ferm temps and sugar/syrup/nutrient doping during fermentation to aging (bulk or bottle).
So, since it may take a year or longer to taste the ultimate results, maybe it's best using the commercial syrups if you're feeling you can't quite get them right yourself. Also buying a pack of D-90 and D-180 will give you a good comparison to what to aim for when trying to concoct them yourself.