I've done all the seats in two cars using foam from another vendor, and have had a difficult time (never done this before) pulling the covers, but turned out good results. I've sold foam sets to others (3 sets) and one person came back and said his upholstery guy could not make the rear covers fit. I showed him my results and I've not heard from him since. The covers are difficult to fit, but persistence and a lot of hand work will give good results.
I would not take my foam to an upholstery shop and ask them to fit the covers as this job is difficult and any problems they had would be blamed on the foam. Let the shop buy the foam or fabricate it.
Similar situation with convertible tops. Most shops are familiar with a certain brand of top, and if you bring one with which they are not familiar, any problems they have will be blamed on the top.