Here is a list I made up earlier showing all the higher weight/height seats available. I am only including those with 17"+ harness height and 40lb+ weight limits. Please note these are US seats. Canada doesn't have all these seats and with some of their seats, they may have different weight limits.
For your situation where you might be installing the seat into smaller cars, probably the radian or the nautilus might be the best choice. Take a look at most of them and decide which one might work well for you.
Convertible Seats (installs both rearfacing and forward facing harnessed seat):
Recaro Como/Signo (~$249-$290 depending on model) - top harness height is around 19 inches (Signo is 18 inches, Como is 19 inches) and harnesses up to 70 pounds (5-35lb rearfacing).
Sunshine Kids Radian (~$199-$278 depending on model) - top harness height of 18 inches. Can hold a child up to 65/80 pounds (depending on the model you buy) (5-33lb rearfacing). This seat folds so it can be carried easily (though it is still a heavy seat to carry, especially around the airports). These seats can use the tether rearfacing. This seat is also pretty narrow when compared to other seats so often a good choice when trying to fit multiple seats next to each other in the car.
Evenflo Triumph Advance (~$129) (don't confuse with the original triumph which is actually a very short harnessed seat)- top harness height of 17.5 inches. Can hold a child up to 50 pounds (5-35lb rearfacing).
Britax Marathon, Boulevard, Decathlon (~$229-$289 depending on model) - top harness height is 17 inches. Can hold children up to 65 pounds (5-33 pounds rearfacing, though seats manufactured in 2008 should go to 35lb rearfacing). These seats can tether rearfacing.
Fisher Price Safe Voyage (~$149) - based on the Britax Marathon shell)- top harness height is 17 inches. Can hold children up to 55 pounds (5-33lb rearfacing). This seat also rear faces so it can be used for babies up to school age. This seat is being discontinued so it is hard to find and might have older manufacturing dates (seat expires 6 years from when it was made).
Youth Seats (installs forward facing with harness):
Britax Regent (~$249) - top harness height is 20 inches. Can hold children from 22-80 pounds. Requires a top tether after 50 pounds. Very big and cannot be taken on an airplane This seat really doesn't work for young children (under 2 who really should be rearfacing anyway) since it is so large.
Safeguard Child Seat (~$429) - harnesses from 22-65 pounds. Top harness slot is 19" high. Very expensive, but a great seat.
SafeGuard Go Child Restraint (~$199) - harnesses from 30-60 pounds. It's harness is a little different and it depends on the seat back. Requires a top tether for installation of seat. Not really the best choice for an everyday seat, but might be good for traveling.
Combination Seats (installs forward facing with harness and then converts to a booster):
Graco Nautilus (~$149) - top harness height of 18.5 inches. Can hold children up to 65 pounds. This seat is forward facing only and converts from a harnessed seat to a highback booster to a lowback booster.
Cosco Apex 65 (~$80-$129 depending on model) (also referenced as Safety First Apex, Safety First Biltmore and a few other names) - top harness height is at 17.5 inches and can harness from 22-65 pounds. After the child outgrows the harness, the seat does convert to a highback booster. This seat does require a headrest behind the seat so it will not work in cars without headrests or tall seatbacks.