First of all - congrats on the weight loss! It makes sense to me that you would feel your sit bones more now.

Look for a bike shop that has a saddle-fitting tool, which is basically a type of "memory-foam" seat. You sit on it, adjusting yourself to make sure you're on your sit bones, and then hold it for a few moments so that it makes an indention. Get up, and there is a little ruler that will tell you your size. It's genius!

Come to think of it... you could probably do it on your own with a piece of the same kind of foam. Just measure from the middle of each indention. Good luck!