I've purchased a few bits and often snap up 2nd-hand terrain when it comes up for sale on the forum. I can make most things to an OK standard myself but time is a problem... and energy for that matter

Have a look at TwoShedsFred on ebay - they do a fairly 'set' range of bits which are generic enough to fit most games and are done to a good standard. For me they are a good balance of price and quality.
There are a few sellers on Etsy but they're harder to find and the quality seems pretty varied.
From the handful of commission-based terrain makers I follow on Facebook they seem to have a pretty regular set of customers as they never seem to advertise for work, they're just steadily making things. As a result I've never directly commissioned anything since I've no idea what they cost! I understand that most won't quote without a chat as each commission is very varied but as a customer that tends to imply "better have deep pockets"

Again that's not unreasonable, it's got to be worth the time & materials cost i.e. not be below minimum wage or why bother?
There's also Adrian's Walls who builds and paints up existing MDF kits as a sort of premium service. From memory I think it costs 2-3x the retail price of the kit (plus cost of kit, naturally). He's often found at wargame shows. I've never purchased as it's just that bit outside my price range really, though again I don't think it's overpriced, just more than I can justify

From time to time I discover a seller on ebay who seems to enjoy making and selling things speculatively but they usually only last a few months or a couple of years before they run out of steam (or get enough regular work to drop the ebay listings perhaps?). They are typically selling at a price I'm happy to pay but which probably doesn't actually reflect the time that went into the work i.e. it's not really about earning for them and more about enjoying the build process and then not clogging the house with the result

All the above is UK-centric in case that's relevant.
Hope this is vaguely helpful!