It depends on how many you need. Less than 100 try Parks Seed or Jung seed. For 1000 see below.
A 'paint' pen you can get in the craft section is very good. And actually on the plastic labels a regular #2 pencil lasts a long time. More than one season.

Here is a link that might be useful: Plant labels

I buy sets of mini blinds at the discount store or if I am lucky someone is trashing some. Remove the slats and cut into the sizes you need.

I buy them 1000 at a time for my nursery. I sell them to customers who want them at my cost plus 10%. Check local nurseries. I also use the paint pens from the craft store.