Those Dexter Torque-Flex 3,500# axles are available at thousands of dealers all over the country. Pretty much most shops that sell and service trailers would be able to put a new axle under your trailer. Axle replacements are quite common. As a suggestion, I would look first at a dealer who sells horse and work type trailers rather than an "RV shop." You'd probably get a better installation and it would probably also be cheaper. Just my 2 cents.
For your Casita
you'll want these specs:
65.5" hub face to hub face, (the outermost points of the wheel brake drums from one side to the other.)
51" outside to outside bracket spacing, (where the brackets actually attach the axle to the bottom of the trailer frame.)
5 on 4.5" bolt pattern, (5 lug nuts on a 4.5" lug spacing pattern for your current wheels.)
EZ-Lube bearing hubs
High Rise axle, with the torsion arms set at 10 degrees down angle