Polydextrose can only be found online. In theory, maltitol and isomalt might be able to found at a health food store (they carry other sugar alcohols like xylitol and sorbitol), but in all my dealings with sugar free/low carb bakers, I've never heard of anyone who's found them locally, so I'd place them in the online only category as well. Inulin can be found at Trader Joes and at some pharmacies under the brand name Fibersure (Proctor & Gamble). The huge downside to inulin, though, is that it's super expensive (~$16/lb.). Purchasing polydextrose online (~$2/lb + ~$5 shipping charges) will save you a ton of money and yield the same results.
As far as online purveyors go, all these ingredients can be found at
Netrition.
Before you start shelling out money for these ingredients, though, it would be a good idea to ascertain your ability to tolerate them. You can find many commercial products that utilize these ingredients- Walmart carries quite a few. The Russel Stover toffees are isomalt based, I believe, so if you're okay eating those, you should be okay baking with isomalt. Maltitol is extremely common- all the Hershey's sf chocolates are maltitol based. To test your sensitivity for polydextrose, I'd buy some Breyer's Carbsmart ice cream. The ice cream contains other potentially laxating ingredients so it's not the best litmus test, but as far as I know there is no pure polyd based candy/confection.
A trip to Walmart and about a $6 expenditure should give you a good idea of how your body reacts to these ingredients. If you have no issues, then I'd proceed with an order from Netrition.
As I mentioned earlier, isomalt will give you the best texture but polyd will give you the best flavor. Assuming you can tolerate it, I would go with isomalt and utilize another ingredient for the caramel taste. Although I'm not a big fan of fructose and it's associated health risks (increased triglycerides/increased insulin resistance, etc.), it is low GI, it does brown well, and in small quantities doesn't pose that much of a threat. I would brown a little fructose (either in the form of pure fructose or agave nectar) and add that to the isomalt. Both are standard health food store fare.