There are lots of 8-bit remakes in the market. Assuming you want an exact copy, the choice of makes gets smaller and smaller, if you want something that is ready to use out of the box and a complete unit (keyboard, case, storage), the choice gets even smaller.
You did not write whether you care for a cycle-exact copy or a "real" 8-bit CPU (as opposed to FPGA re-makes or even software emulation). That makes your question a bit broad, and some people are a bit picky whether a Z80 computer that doesn't use the "real thing" can in fact be a Z80 computer. I am listing a number of Z80-based re-makes (software, FPGA, Z80) below:
- In case you are fine with FPGA-based hardware, the MiST is a nice option; it uses whatever USB keyboard you want, outputs VGA, and has Atari-style joystick connectors for retro peripherals. Existing loadable cores for this platform cover most of the classic systems from the 80ies, both 8-bit and "better". This seems to be an advantage, but in my personal opinion is rather a disadvantage: The amount of cores available on this platform makes concentrating on one specific system extremely hard - I constantly find myself testing other cores instead of concentrating on one and tend to get lost in the variety of options. A large part of retrocomputing is "living within the limits" for me - The MiST pushes these limits too far, IMHO, by being able to "be" nearly anything. But, as you don't seem to have decided on a system yet, a platform that can be any system might fit well.
- A Harlequin is a nice option for a ZX Spectrum re-make. It uses a "real" Z80-CPU and is a close copy to the ZX Spectrum 128 or late Amstrad Spectrum models. It is, however, not available as ready-to-use item, but rather as a kit and requires soldering (but see the "Omni" below"). Re-made ZX Spectrum cases can be had from Retro Radionics.
- In case you are fine with pure software emulation, a RetroPie might be an option - This is platform based on Raspberry Pi hardware and can emulate quite a number of 8-bit systems in one single distribution. Very similar to the MiST, even if totally different technology, but also with the same downsides, but very probably the cheapest option to get into retro-computing.
- The Omni 128 is a ZX Spectrum (Harlequin) clone complete in a case, ready-to-go. They are said to have quite long delivery leadtimes, but may be an option. The Laptop advertised on their website doesn't seem to be available.
- The Spectrum Next is one of the most ambitious projects in this space: An FPGA-Based ZX Spectrum re-make, beefed up with mass storage, HDMI-output, lots of hardware-add-ons in the FPGA, and plans for a very nice case designed by the late Rick Dickinson, the original designer of the ZX Spectrum. They have already delivered mainboards to backers, but the ready-to-go version seems to be delayed. The Next was a Kickstarter project, whether it will be available for non-backers is not entirely clear to me, although they have set up a web shop.
- In case you are happy with a plain-old CP/M 2.2 system and prepared to solder, the RC2014 is another option. It does not really copy a "classic" design, but rather is a modern re-make of the "typical 1980-ies CP/M system" on modern hardware (but on a "real" Z80) - Available in kit form, requires soldering. Can be bought here.
- The ZX Uno is another FPGA-based ZX-Spectrum that can reproduce the complete ZX Spectrum range from early 48k models to the 2+. It comes with SD card storage, various extensions to the original and a 3D-printed case. Needs a PS/2 keyboard and a VGA monitor, then you are ready to get going.
- If you are prepared to solder, the ZXmore is a very cheap option of a computer kit that can emulate the Sinclair ZX80, ZX81 and ZX Spectrum (monochrome only). The kit is frequently offered on sellmyretro.
Most of the above are enthusiasts' projects with supposedly more love and entusiasm than professional entrepreneurship going into them (though all of them seem nicely engineered and are said to work perfectly). Before buying something from the linked web-shops, you might want to inquire for terms, conditions, and especially availability and lead times - Some of the offerings are only built-to-order. Most of the above platforms have forums that provide user-to-user support, and I recommend you make yourself familiar with these before buying anything. There have been failed projects in the past that collected money from buyers and showed up as unable to deliver (The infamous ZX Vega plus project needs to be mentioned here. But consider this rather an exception than the rule).
