I went thru the same hunt for some Anglo-Boer War oxen pulled cannons. Plus, I'm an original soft plastic kind-of-guy. So, I settled on the Atlantic Originals/Nexus-Atlantic Re-issues sets because (1) they were soft plastic so easy to work with, (2) I could get them fairly easily on-line, and (3) you get 8 or more per box = good value! The new Nexus re-issues are only about USD $8.00.
Regarding Nexus (Atlantic re-issues which are also out of production now), go to the link below and check out:
Original Atlantic also made a Pioneer Wagon set with 4 wagons and 8 oxen. I have one box. The oxen are nice, wagons so-so but now collector's set so eBay or ? = $. Go to this link to see a scan of this, scroll down:
I think some of the newer American Indian sets you only get 2 oxen per box, and the harder plastic is difficult for me to cut with an X-acto knife. Check PSR's "Manufacturers" Link, and go to the different makers to help make your own decision.
You can see some of my Anglo-Boer War conversions with oxen in the Strelet's "Contributions" link dated August 8, 2006 Anglo-Boer ...
By the way, the Airfix Wagon Train set's horses are a pretty small, but I still love the wagons. So I have bought up 25 boxes of the new re-issues and have made Anglo-Boer War conversions using Atlantic/Nexus Oxen, Airfix Wagons, and Imex Wagon tops and accessories. I've got (1) supply wagons - 3 types, (2) chow wagons, and (3) ambulance wagons. I hope to be able to complete the project for S*R's Contributions fairly soon. Good Luck!
Forgot the horses. I think the best horse/wagon set for sculpting and detail is Italeri's Napoleonic set. The wagon is pulled by 2 horses, but the box has 4 horses in it. 2 extras for conversions. So pretty good value. Produced in Silver and Blue that I know of. Here is PSR's review link: