You usually find an outdoor outlet on most homes. That's your best bet. Garage would be a second choice but most homes by me the outside outlet and garage may be on the same circuit, so be careful and ask the client if they have any big appliance plugged in inside their garage. A lot of people by me have like an extra freezer in a garage. But always ask before plugging in if they have anything on the circuit that you may be sharing power with.
I know this is an old post that I originally started, but I wanted to give an update as to what I ended up purchasing and it works out great. I purchased a Rooftop Dometic Brisk Air RV A/C. Cools my 6 x 10 cargo trailer great. I did, however, had to make sure my trailer was insulated well. I use two electrical chords: One chord is for my A/C unit (it's a very thick chord to handle 15 amps). The other chord is for my lighting (fluorescent), force dryer, clippers, radio (this chord is not as thick as the a/c chord). I tell my clients that I need an outlet that can handle at least 20 amps. If their outlet uses less than 20 amps, then I have to plug each chord in different outlets that are on different circuits. This set up always works. If for some reason I am only allowed one outlet, I can temporarily turn off my A/C during the dog drying process.
I purchased the Dometic 13500 BTU Duo Therm Brisk 2 RV A/C and I live in Florida.