Home made mustard is actually pretty easy - seed, powder, vinegar. You can alter the profile of what you want by type of seed and vinegar, add herbs or spices, etc. Lots of internet sites out there with recipes and sources.
I'm a mustard guy - 4 different types open in the refrigerator - another 10 or 12 on the shelve in the pantry.
When I have "dregs" in the bottom of the jar or bottle, I'll rinse with a little vinegar, add some spices and / or hot sauce and make a mmustard based bbq sauce - I also have a "mother" bottle of this sauce in my refrigerator that I just keep adding to.
Two of my favorite mustards are - my favorite, imported from Cleveland, Stadium Mustard - this is my "go to" mustard and bring back 4 -6 bottles whenever I go to northeast Ohio and Datil Mustard, from Datil Do It, in St. Augustine, FL. If I'm in the St. Augustine area of Florida, I'll bring back 1/2 dozen jars. I've also ordered by the case from Datil Do It a couple of times.
* IHP Cactus Farm & Sauce Shack *
18 1/2 & 22 1/2" WSM
22 1/2" OTS
26 3/4" OTG