Exped do a range of olive green dry bags with handles on the bottom. Not always the easiest to find in that colour, but I have been happy with those I have used. Heinnie are currently out of stock, but Tamarack look like they can help.
I have never put my tarp in a dry bag, can't think of anyone I have travelled with who has either. I have just used a silnylon stuff sack or similar and lashed it to the outside of the pack, or had it in a top pocket. How does using a dry bag compare? I can imagine both pros and cons, but would be interested in hearing actual experiences.
I have to admit for the price and the stigma the karrimor dry bags do the job very well.
I put rolls of toilet paper in one and gave it a very heavy showering for ten minutes with no ingress
I also filled one with water and left it hanging for a few days with minimal leakage.