Muriatic acid is hydrochloric acid at about a 25% concentration.

Any janitorial supply store will sell GELLED hydrochloric acid at a 12 to 17 percent concentration for cleaning toilets. Just ask for a hydrochloric acid based toilet bowl cleaner.

The toilet bowl cleaner will come in a quart size bottle and the fact that it's gelled will make it stick better to smooth and vertical surfaces. It'll take longer to work because it's half the strength, but it's also not quite as hazardous to work with.