To address the original poster's question, I think a simple butt plug of a standard diameter would be enough to get relaxed for anal sex. Maybe wearing it for a few hours a day, for a few days in a row and on the same day prior to having anal sex. Then again, I have never been on the receiving end of anal sex. However, this is the first plug I ever bought and it seemed to relax me on the days that I used it. I have also found the shape to be very easy to insert and remove, and pleasurable:
http://www.amazon.com/Luxotiq-Anal-Plug-Stainless-Steel/dp/B007F17OG0/ref=pd_sim_hpc_4
That is the aluminum version. There is a lower priced, but slightly higher quality stainless steel version of exactly the same shape and size, that I would have bought instead if I had known about it at the time I placed the order:
http://www.thechaingang.com/adult-adventure/female-body-jewelry/butt-plugs-probes-insertables/art-deco-butt-plug.html
The narrower neck of the plugs above allows you to leave them in as long as you like.
Here is a nice looking set of dilators- they have a wider neck, for short term training. The wider neck leaves the sphincter muscles open the entire time the dilator is inserted, so you would probably be very relaxed after wearing one of these for an hour or so, maybe less. The largest size dilator could take a few weeks of consistent training to work your way up to if you were so inclined. However, I am sure the smaller sizes would still prepare you for anal sex.
http://huse.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=HDC&Product_Code=21847&Category_Code=A3#.VOZ99ULjh
I am not affiliated with any of the above sites/retailers.