When using a tumbler (which I really much prefer a nice English bone china teacup) I have found that plastic just won't do! The tea just does NOT taste the same at all. After much (and I do mean much) trial and error, I have found some tumblers that work well for me when I'm drinking tea on the go (or it isn't practical to use a cup and saucer)
My newest tumbler is from Teavana. Like everything at Teavana, it was a bit overpriced, but I will say that I am happy with it so I find it is worth it. The inside is all ceramic- which I find to be right up there with drinking from porcelain. I find it easy to take this tumbler in my car and drink tea on the way to work or running errands.
The other tumbler I've found that I like is the stainless one below. It has many advantages, but a few disadvantages. Advantages are that you can put your tea in and leave the house right away because when you are done steeping you can push the plunger down and it stops the infusion. I used to have a Bodum press tumbler that was plastic- but besides it being plastic, you would waste a lot of tea at the bottom with the leaves. This one you just put the leaves in the infuser (which goes all the way down to the bottom) and when the proper infusion time is reached you just push the plunger and voila you have tea. The only problem is it is too hot to drink, and stays that way for hours! This is an advantage if you want to take it with you for the day and drink later, but if you are driving to work, you can't drink it on your drive!