How to replace Bouillon cubes without making them on my own

The examples you gave - pasta and rice - are presumably using boullion cubes and water. The cubes are basically supposed to be dehydrated broth, so you can just use whatever kind of stock or you prefer instead of the water. Often boullion cubes are saltier and have more umami than the stock they'd replace, so you may find you want to add back in some salt and some other source of umami (MSG or a more substantial ingredient), especially if you're using vegetable broth.

Depending on what your objection to bouillon cubes is, you might also like a vegetable stock base (Better Than Bouillon is a common brand). It's basically the same thing as the cubes, except in paste form.

But in any case, the bouillon is basically stock (minus water) so you should look to stocks for your substitutions. The primary two concerns will be salt and umami, but you probably are benefiting from some more subtle flavors too - and even if the bouillon cubes are lacking in that department, a good flavorful stock will be a welcome improvement.

The packaging for your favourite bouillon cubes should have a list of ingredients that will give you a place to start. Theoretically they are dehydrated stock, but unless it's a particularly fancy brand, they're usually mostly salt. Using wine, or even just water, is often a fine substitute if whatever you're cooking has plenty of its own flavour, though you might want to increase the salt. What else you could be missing is the savoury aspect that would come from the vegetables or meat that would go into the stock. You could possibly make up for this with something like Worcestershire sauce (not vegetarian). If your dish doesn't have much in the way of onions or herbs (common stock ingredients), then saute some onions at the start and add some herbs at the end to substitute.

If adding a commercial sauce isn't as "natural" as you'd like and the onions/herbs isn't the issue, I'd suggest reconsidering just making some stock and freezing it in ice cubes or silicon muffin cups (then pop out into a bag to store). If you are vegetarian, making stock IS actually pretty fast... it really only takes 15 minutes of boiling some veggie trimmings to get a tasty stock. If not, while it does take time to make meat-based stock, very little of it is "active" time and you can make a LOT in one batch (assuming you have freezer space to store the results).

You could make a little vegetable stock. Onion, garlic, carrots, celery, herbs, and a decent amount of salt. That should replace both the liquid, flavor, and salting functions of a bouillon cube.

Mind you, making stock takes TIME, but not much effort. You can also make use of veggies that are a little past their prime that you'd otherwise throw away.