I have ordered from Mike's Wet Pets (off of aquabid since I don't buy as much off of Ebay) often and really like his plants. I have never had any issues with his plants. His bunched plants have a nice amount of individual plants to each bunch and his single rooted plants have very good root systems.
I have also bought from plantedaquariumscentral.com. I found a lot of plants that I couldn't find anywhere else on there and the plants came in pretty good shape. No complaints and I would more than likely order from them again in the future.
My favorite here lately is www.bamaplants.com
. His sn on aquabid is NoFishToday. I have ordered tons of plants from him within the last couple of months off of aquabid and from his site. His plants are always very nice and I have not had any transition issues from any of the plants.
I also prefer to buy from other hobbiests when they trim their tanks because it helps them out plus you normally get nice and healthy plants.
With hitchhikers--most plants (unless otherwise specified) are a normal part of buying plants. Taking the plants and dipping/swishing them in a light bleach water solution will kill off any hitchhiking snails or other possible disease. I have gotten snails, tiny shrimp, and know of people who have gotten fish fry/eggs as hitchhikers.