I know of a number of ladies that offer 1/2 sessions. They may not widely advertise to the public that they do, but I'm sure a quick note to a lady asking if she does would be acceptable.  

Emma.

Here is what I suggest.

Just do a search for the type of provider you would prefer.  Then check out their profiles and see if their ads or websites indicate if they do half hr sessions.  If a provider really intrigues you and does not appear to offer half hour session just contact her and ask.

Sessions are priced better for buying in bulk (the longer the session the cheaper it gets on an per minute basis).  Not sure if you want half hour sessions to save money or if you just want to test drive.  I generally prefer hour long sessions since I like to play and get some quality time.

As others have chimed in, search through the reviews of the ladies who fit the looks you're searching for; then email / call / text them one at a time. Wait for x amount of time for a reply before email / call / text... when they reply, ask them in a nice way if they do 1/2 hours. Maybe you have time constraints not mentioned here for whatever reasons.    

Good luck in your search and get a vip membership so ladies can send you mail. (I'm not a vip, may not review anyone new anymore for the "Free VIP" membership. If you wrote to me, I will never get a chance to read it. My apologies.)  

(full disclosure, I have found my atf, she might be or might not be reviewed by me.)