Craigslist

I find the majority of furniture I use on craigslist, and the biggest skill you need to aquire to succeed there is persistence. It only takes a few minutes to do a targeted search, so I will search at least once a day for whatever I need until I find it. . I usually limit my search to furniture by owner, under a certain price, with an image. I also keep google maps open in a separate tab so I can search for locations as well. It can be a pain to fall in love with a table, have the designer fall in love with a table and then realize that I have to drive an hour to pick it up. Any item that I think is interesting I send in a link to the set designer with my thoughts ("this one is my favorite", "this might work if we repainted it" etc.). If I am worried about timing I sometimes call or text the designer as well to let him know to check his email.
Once I get a reply from the designer I email the seller. I get the best response from sellers when I am brief (no need to tell them who I am or why I need their chair), I include my phone number and email address, and I include a list of times when I would be able to pick up the item over the next few days.
An exception to the brief post rule is when I can't afford something. If I have come across something unusual that I need, I start using the other bargaining tools in my bag of tricks. I can offer the rent the item, borrow the item and advertise in the program or with a sign in the lobby about where it could be purchased. If the theatre is non-profit I have the extra incentive of providing a letter of donation for tax purposes. I have offered comp tickets before and have even offered tickets to other shows (this spinning wheel is for theatre X, but the seller lives closer to theatre Y where I just designed a show and was given two comp tickets that I wasn't going to use).
Sometimes people don't reply. Sometimes you email 4 different sellers and they all write back to say the item was taken. In the end persistence is key. I trust that the item will show up, and it usually does.
To make my records neater and more professional I purchased a receipts book from Staples for around $5. I used to hand write a receipt on whatever scrap of paper I found in my car. This little book is simple, and I often don't know quite what to write in some of the blank fields, but it looks much more professional and is far less likely to disappear when I am cleaning out my car.
Always bring cash, and make sure you have exact change. I recently paid $39 for a table and chairs that was posted for $35. I stopped at an ATM on the way to the house, got $40 and the seller only had $1 cash on him to give me change.