Well, don't go by me, but....
I have access to the material, equipment and helpful friends to make just about any mortar or cannon that could be transported in a pickup truck, and I've learned how to do much of the required work myself (And I've made a few, usually as part of a collaborated effort among like-minded individuals). That said, I've found that even with the required resources at my disposal, making them from scratch can be somewhat "false economy" if everything (Especially the time involved) is taken into consideration. Just ask anyone who's built one how many hours they have into their project.
There are several builders of cannons/mortars today that I look at, scratching my head saying "How can they make a decent profit at what they sell their stuff for?" There are some really good makers that offer excellent "bang for the buck" today.
The above said, I wouldn't trade my experience of actually making stuff myself for buying it. I've learned a lot in the process. If I were younger and had the time available to do it again, I would.
You can of course buy some parts "off the shelf" and do some of the work yourself - I recently bought two mortar bbls on ebay that need bases made.
Also, when someone asks " Hey, where can I get one of those?", it's kind of satisfying to be able to say - "You can't buy one exactly like this, I made it myself."