3/8" is the most commonly manufactured size. You can still find 3/4", but you'll pay much more for it. It's not common at all.

There's quite a bit of mid and higher end 3/4 inch engineered being manufactured today, but finding it in your average flooring store is slim. Ten years ago it was non-existent. With concrete slabs their reasoning is probably the fact it needs a very flat sub floor, particularly with direct glue downs which does increase time and prep costs. And maybe they've had some failures as some guys really don't know the proper way to prep (flatten) concrete.

As far as costs, there are some 3/4 inch prefinished engineered that are actually priced better than Mirage and Lauzon. Both are great products, but you pay through the nose as too many get a piece of the action. IE: distributor, dealer. You won't find those annoying short lengths either and the overall specs exceed Mirage and Lauzon in the wear layer. Lengths are actually longer than solid hardwoods too, going to eight feet.

A few drawbacks with 3/4 inch engineered on concrete are the vertical height differences when it comes to adjoining tiled surfaces. Not so much with new construction as it can be pre-planned. With remodels it can be a bit demanding with baseboard heights, but today most are replacing shorter height baseboard with higher profiles. As far as going up against carpet that can easily be corrected with shims (underneath) on the carpet side.

As far as availability of these 3/4 inch products, it's usually a 3-5 week lead time. Typically the order minimum is 500 square feet. Made in the USA.

Carpet Shims - FFWD video to 1:20. They can be glued to concrete.

Thanks for the responses. Our home was built in 1965 and was originally carpeted. The slab was not leveled very well either. I can certainly tell a difference when walking on the 3/4" flooring over the 3/8". I'm not a fan of the transition strips and have considered replacing it all but it would be a 2,000 foot redo.

I have some sample pieces of high grade 3/4" thick engineered flooring from the 60s or 70s. This was more of a boutique item to my knowledge. We would buy it to do borders around classic parquet patterns. Now there are more options in high quality engineered flooring.

Look into Owens engineered flooring. They make many products in 5/8", which might suffice with a little floating where it meets the existing. According to their price list, Owens will make 3/4" for an upcharge:

https://www.quanex.com/Products/Residential/Wood-Flooring.aspx

Also Shamrock has 3/4" listed on their website:

http://www.shamrockplankflooring.com/products/environeered-flooring/engineered-34-flooring/