If the person feels they are a danger to themself or others (suicidal or homicidal thoughts), they can go straight to an ER. Large hospitals generally have a psych ward, or at least a supervised ward that the person can stay in until they can be placed in an appropriate facility.

If the person feels it is not an emergency but they need to be somewhere supervised where they can get help, they can search "psychiatric hospital" or "in-patient mental health services" online, and several options should be available near them. (Depending on where they live, they may even be able to search a site like the NHS to find an appropriate place.) At most in-patient clinics, a person can voluntarily check themselves in and start getting help right away.

If the person feels they may just need one-time immediate help, calling a hotline is a very good option. Searching "crisis hotline" or "suicide hotline" online will give the person a list of numbers they can call anonymously if they are very stressed, anxious, suicidal, considering self-harm, etc. But if the person still feels they need help after this, they should absolutely consider going to an ER or checking themself into a clinic.