the original 18" solid plastic steering wheels where made by sheller part number D-48140.
sheller also made steering wheels for the 5 ton 800 series trucks (eg=m813 truck). these steering wheels are almost identical to the halftrack steering wheels. to use one you will need to machine the center shaft bore to the correct taper. below is a pic of an original steering wheel and an 800 series. also notice the horn button parts will be the same.
horn button parts. do a search for "m35 horn button" on ebay. also do a search for "forklift horn button" now you will have all the horn buttons you need.
the steel spoked steering wheels. the only thing i have found for these are the ones used in the Farmall A, Super A, B, C, Super C, H, Super H, M, Super M tractor. you would need to keep your old steering wheel hub center and weld it into the center of this steering wheel.
the link to the case steering wheel also requires machining the center, im not sure how well it matches but its a nice looking steering wheel.
http://farmallparts.com/products/?view=product&product_id=24161 http://www.ecrater.com/p/9048602/case-tractor-steering-wheel-cks090-430-530 
