Anodes with petcocks are available at Camping World. What you don't get with that smaller opening is the flushing of the crud that accumulates in the bottom of the tank, so you'll need to back-flush the heater. Personally, I don't use the petcock version for that reason.
You replace the anode when it's been consumed, or when yiu feel it's time. Personally, I replace the anode when I flush - and I flush it about once a year, or when the hot water starts to smell or taste funny, whether or not it's been consumed.
I'd put the anode in after the flush - it's one less thing to have to mess with before you head out the next time.
al
