For markers I just raided my 4 year old daughter's collection of foam shapes. They are roughly 1- 1.25" across and 1/8" thick. They come in lots of shapes (heart, triangle, square, circle) and colors. We put them under the mini to show status. Red hearts = bloodied, dark green circle = Hunter's Quarry, light green circle = fey warlock's curse, blue circle = Fighter's mark, yellow triangle = slowed, etc.
You can get a tub of thousand's of them (more than you'll ever need) for $8.
Here's a site that sells them:
http://craftsuppliesforless.com/kidscraftsupplies_foamies1.html
The ones I use are the "Foamies Bucket of SHAPES, Assorted Shapes - 5.5 Oz"
There are bunch that are too big or small to use, but there are plenty of useable ones. They are very effective and cheap. You should also be able to find these in local craft stores (Michael's, Hobby Lobby around here).
John