Yup, many folks here use one of several authorized ticket resellers, including
Undercover Tourist, GetAwayToday, LastMinuteTravel, or aRes. I think LMT is the one that doesn't let you add MaxPass ahead of time, but it's so easy to add once you're there, that doesn't make a difference to me. Also I don't necessarily WANT it ahead of time for my entire stay, because my first and last travel days are short enough that I won't really need it.
Just for grins, I just now went and researched current ticket prices. I looked for 4 day parkhoppers, both with and without MaxPass. Here's what I found:
Book through
Disneyland: $380.00 w/MaxPass (+$60) $440.00
" GetAwayToday: $360.00 w/MaxPass (+$60) $420.00
" Undercover Tourist $350.95 w/MaxPass (+$60) $410.95
" aRes Travel: $348.70 w/MaxPass (+$60) $408.70
" LastMinuteTravel $336.00 (can't add MaxPass ahead; but add daily for standard price of $15 a day-+$60 = $396.00)
So for a standard 4-day hopper, LMT saves you $44 over Disney, aRes saves you $31.30, UT saves you $29.05, and GAT saves you $20.
By the way, LMT has the world's clunkiest website. Find a code for a "free membership," get signed in first, then search for "Activities" rather than putting in your destination. Once you get to the Activities page, THEN put in "Anaheim" and your dates, and DLR tickets should show up.