I’m an auto repair professional. I have my own shop. I strongly advise against it.
Let’s start with the actual problem. “CV joints are cracked” makes no sense. CV joints don’t crack. CV boots can crack, but there’s a big difference between replacing CV boots and replacing CV joints. Understanding this critical difference and using the correct terminology is not simply an exercise in pedantry, it’s vital to properly evaluating what needs to be done. Getting it wrong can easily lead to spending money on repairs that aren’t needed. If you don’t fully understand what needs to be done and why (and part of that understanding is knowing how to express it accurately), you’re setting yourself up for wasting money.
If a CV boot is cracked, then the joint inside will lose some grease and get contaminated with water and dirt. This will shorten its life. Decades ago, it was wise to replace the boot to maximize the life of the joint, as the repair was to replace the joint with a new one, which was rather expensive. Things have changed, though, and now the repair is to replace the whole drive axle with a remanufactured one. The reman axle has remachined joints and new boots (two per axle), yet costs less than one new joint. On most cars, it costs at least half as much to replace a boot as to replace the axle, and in my opinion is not cost effective. I usually advise my customers to forgo replacing the boot, wait until the joint actually starts to go bad, and then replace the axle. It is typically quite a while, often a year or two, before it gets to that point.
If a CV joint is worn (it’s virtually always an outer joint - the inner ones have very little stress), the typical symptom is a rapid repeated clicking noise under acceleration in a tight turn, like a parking lot maneuver. Eventually the joint will fail, at which point the car won’t go - it’s like being stuck in neutral. But again, I’ve seen cases where the joint can last over a year after the clicking starts.
In sum: if the only symptom is a torn boot, I’d let it go; if a joint is worn, it’s probably not urgent, but is probably wise to fix it before heading on a long trip.
Now on to your actual question. The longstanding convention in auto repair is to generate some profit from labor charges, and some profit from markup on parts sales. But what’s being sold is a repair, and while its labor and parts components are priced separately (unlike in a restaurant), that’s for accounting purposes and not because they are meant to be sold separately. Any shop with decent business sense is not going to forgo the parts profit - they’re either going to refuse the job or bill enough under labor to maintain their profit structure. And this means there’s no savings to be had, just some extra work on your part.
Pricing aside, there are other potential problems with using customer-supplied parts. As mentioned above, you’ve got no warranty. All too often the parts are wrong for the car, or of poor quality. A good shop wants to do a good job and guarantee it, and to do so they need to be in control of acquiring the parts. Short-circuiting this process tends to generate problems, not savings. It’s just not a wise approach.