"Supposed to" is correct.

Example:  We were supposed to go on a trip.

"Suppose" is a thought or an action ( "to suppose").

Example:  I suppose there are different flavors to taste.

Supposed is a past participle verb so it ends with "ed."

The same rule applies when using "used to."

Add the "ed" and you should be good to go. Hopefully this helps.  If not, there is always Plan B: Find another way to say it!