I use the info off the supermarket packaging when available, it's pretty accurate. Use the info on the back under Nutrition, not the summary on the front. Otherwise I use the Fat Secret website, which is UK based. You have to be careful with US sites as they measure things a bit differently, including fibre within carbs for one thing. Also, their portion sizes are a bit odd (when given) and they prefer to do things by portion rather than per 100g.
Foods I eat frequently, I log in an Excel sheet for easy referencing. As long as I have my phone with me, I can pull it up anytime I need it. It looks like this:
Name of food - carbs per 100g - fibers per 100g - serving size in g - last field calculates how many carbs are in the food for that serving size. (Easily adjustable too! I just change the serving size and bam.)
I fetch the info from the packaging and otherwise get a fairly good estimate from Google on how much is in unlabeled foods.
I've download the "carbs and caps" app on my phone. Very recommended- it's so easy to use and works it all out for you. It's a cost of £5 but it's proved invaluable for me.