I use one of 3 depending which one I pick out the bag first;
St Andrews marker
5p
USAF Lakenheath golf course marker(great memories playing americans for steak dinners back in the day!)
A few years back I lost on a course a Sterling Golf Club marker that I had for years and used for many rounds,I was GUTTED .
I just put a 5p and 10p coin in my pocket before leaving home. If you are some distance from the hole a small coin can be hard to see from beyond the hole so a tee is a good idea.
the first marker i come across in my bag. one of these days im going to give the bag an oul clear out. maybe before the new season starts. my nephew knows he can come to me if he gets peckish during a round. my bag becomes like a rolling tuck shop during the summer. chilled drinks, the lot.
Can't believe that there are golfers out there still using tees. Sometimes if playing other courses, especially great ones, I would buy one as a momento, but usually somebody brings me one back from their hols, couple really nice ones, but I have one from Kloof Country Club in SA which a dear friend bought me and has passed away. So I will use that more in memory of him.
yeah, i always buy one when visiting a new course! i use a tee if we are just out hacking around. in competition i will always use a proper ball marker.