3P's

Most schools will let you take classes as a non-degree student although they might have limits on which ones. Schools that have multiple APRN specialties sometimes have sections for specific specialties for pharm and/or health assessment and might restrict these to students enrolled in the respective programs. Many schools also have a clinical component to health assessment which typically requires more extensive paperwork that is done for non-degree students. It doesn't hurt to ask though.

Is there a reason why you want to take only these classes? If you're doing it to test out the idea of going to grad school, they I would choose a school you would be interested in attending if you made the jump. If you're doing it to just to increase your own education, any local school should be adequate. I would avoid taking them online unless you're pretty sure a) it's a school you might want to attend, b) they are quality courses that offer more than testing you on material you teach yourself.