We bought our current 5 point harness 4.5 years ago, at Wal Mart. It's a Safety 1st Vantage. I love this seat, and it has served us well through 2 children. But now that our youngest daughter is outgrowing it, we've had to get her a new seat. The Vantage only goes up to 40lbs/40" as a 5 Pt harness, and she is also too young to be in a belt positioning booster seat.
I did my research, and since we're on a budget, I decided to go with another Safety 1st Seat. I really like their products, and since this seat held up so well, I figured another one would. I purchased a new Safety 1st Apex 65 seat from Albee Baby. The ladies over at Car-Seat.Org were very helpful in giving me any sort of information I needed on this seat, including the height of the top tether strap (17.25"!!). So if you're looking for a good, AFFORDABLE seat, this one is available on Albeebaby for only 94.94, including UPS Ground shipping. If you buy it elsewhere, expect to pay upwards of $130 US for it.
What I love about this seat is that it puts the child in a 5 pt harness till 65lbs and 57" tall! I don't think I'll keep her in the 5pt harness till she is 57" tall, but at least till kindergarten. That's only 2 years away anyway! Right now she is 40" tall and 38lbs, and you have to get a new seat once your child has outgrown either of the height and weight limits. I don't think she'll outgrow this one for awhile. Once the 5pt harness has been outgrown, the seat can be used as a belt positioning booster to 100lbs. And it also uses a LATCH, which we have in our van. Our LATCH is only good for weights up to 48lbs, and after that only the tether can be used. And thankfully, we have both!
I know my husband thinks I'm a little crazy for wanting her in a 5pt harness still because she's 3.5, but I just feel safer that way. We didn't know any better with the older 3 kids, and put them in a booster quite early. His reasoning for putting her in a belt positioning booster seat was just because we did it with the other kids, so why not her? I don't exactly recall how old the twins were, but it was a lot younger than they needed to be. We didn't know any better. And with our middle daughter, she moved out of the current seat once she hit the height and weight limit, right around 3.5 too if I recall.
At least now I have peace of mind, and I'm happy with that.