I use white vinegar all the time and have been using it since reading the original tip I think in Organic Gardening magazine decades ago.

I use about two tablespoons per gallon, and it works really well with gardenias used with fish emulsion. (However, the one I currently am growing is not as acid sensitive as the ones I had at my other house. Its grafted on a species rootstock.) It also works great with my citrus and keeps them green (I have a potted lemon and a potted kumquat.)

I would try this route. All you're doing is gently changing the pH of the solution to encourage the plant's roots to take up the nutrition. If its a potted plant, its good to gently give it some fresh acidic soil (premade or make your own from good potting soil and leaf mold with a touch of peat moss,) at least every other year. If its in the ground, mulch with something like leaf mold. Leaf mold is hard to find, its partially decayed oak leaves mostly, look for an ethical source/company when purchasing and ask your local store about it. Use sparingly and do not waste. Its not extremely expensive, but it is a little expensive at about $7-10 bucks a 2 cu bag.

I also like worm compost. I use the compost I make myself from starbucks coffee grounds. My acidic plants love it and in pots I encourage redworms to live and stay. Acidic plants in general (the ones I grow,) seem to love nice humus conditions and lots of microbial activity and redworms promote this. Mine live happily in the pots and if they become unhappy they will leave, but create delicious soil in the pots if given a little compost, leaf mold, coffee grounds or organic fertilizers to work on. (They actually eat the microbes digesting all those products.)

If you have plants of special concern, maybe we can try and address them specifically, some can be picky (gardenias! I especially recommend mulching with leaf mold, some coffee grounds or rich compost, and worm castings/with some worm eggs/worms in it to work the soil.)

If you're lucky enough to live someplace with lots of deciduous trees, you can create your own leafmold and collect the neighbor's leaves, bag it, and let it rot over winter in plastic bags. This creates a delicious acidic leaf mold... I've done that before, but my favorite source cut down all their trees recently. :( ) If you can create it yourself, its absolutely the way to go.

And... if what ashkebird said didn't help, MiracleGro actually makes specific fertilizer for acid-loving plants. Try Miracle-gro.com :)