You can try John W. Negus Co. in Portland. https://www.johnwnegus.com/
You could take them a caster and let them closely match it to what you are wanting to do. 12007 NE Marx St, Portland, OR 97220 They are in building 5 They've been in business for 70 years and can probably help you find a caster that meets your needs well.
Otherwise, if that is not your location, find a bearing and caster supply house near you.