Should you buy an off the shelf passive crossover? This question often gets asked on audio forums. My answer to this is an emphatic "NO!" It's never a good idea to buy one of these units, unless you want to learn by experience the lesson you can learn here for free.
A crossover need to be a custom design. A generic crossover fails in two ways. Firstly, it will fail to do what it attempts to do 99% of the time. Secondly, it omits many necessary aspects which require the box and parts to be chosen first. The result is that your DIY project will fall short of a commercial speaker at the same price.Generally a DIY speaker if well designed will match much more expensive speakers.
My two points here will make more sense with further explanation. Let's consider the two main aspects of passive crossover networks.
1. Dividing networks
The main function of a passive crossover is to divide the full range signal into separate bandwidths for each driver. This is the only function provided by typical generic crossovers and they do even this aspect poorly. One reason is they don't work is that the driver acoustic response adds to the equation. They assume that if you add filters at 3kHz to a tweeter and woofer that the crossover will happen at this point. Here is an example:
It's necessary to choose the filters to suit the drivers, and the actual filters will usually need to be applied at different frequencies for each driver.
2. Impedance and contour networks
The above example uses active filters, which are simpler to implement. With passive components, the performance is also influenced by impedance which varies with frequency. Often impedance correction is performed to flatten the impedance curve so that the drivers will response to the dividing networks in a more predictable way.
A good crossover design will often need contour networks as well, and this aspect is especially driver-specific. Often the response of a driver needs flattening. It may be a notch filter to tame a peak. Sometimes with crossoverless full range drivers, contour networks are still needed.
A decent crossover will also feature an allowance for speaker placement and baffle width. This is called bafflestep compensation.
One size doesn't fit all
I hope that by now it's clear to you why an off the shelf crossover isn't going to do the job. There are just too many things a crossover needs to do that can't be done in a generic unit. At this point you have two good choices. One is to learn to design your own - be ready for a learning curve. The other is to let someone else do the design and get to work building a speaker that has already been sorted. There are plenty of choices available.
A third option is to design your own but make it a little easier by going active.