Check out https://www.mono-project.com/download/stable/ and follow the instructions to add their repositories for Ubuntu 18.04. After running the apt update step, you will have those versions of mono available to install via apt.
You can also browse their repository manually if it's easier for you to find what you want. Just direct your browser to https://download.mono-project.com/repo/ubuntu/. If you're not familiar with how apt repos work, you can look under https://download.mono-project.com/repo/ubuntu/dists for index files. These are text files that list what packages they have made available for a particular distribution/version. Each block of text is like the output from apt show, for example,
Source: mono-basic
Version: 4.7-0xamarin3+ubuntu1804b1
Architecture: all
Maintainer: Debian Mono Group <pkg-mono-group@lists.alioth.debian.org>
Installed-Size: 469
Depends: libmono-corlib4.5-cil (>= 4.0.0~alpha1)
Homepage: http://www.mono-project.com/Visual_Basic
Priority: optional
Section: libs
Filename: pool/main/m/mono-basic/libmono-cecil-vb0.9-cil_4.7-0xamarin3+ubuntu1804b1_all.deb
Size: 149176
SHA256: 6ff813170a85dd11f210c70a2ebe5cd6c9d7b964fe4f54addc24d54e58fe5095
SHA1: d72e40410337822020cc191a575592baa7cf309d
MD5sum: 6474364d238ddf60e8817ef4c0300114
Description: Visual Basic 2012 Cecil libraries for Mono
This package contains the Cecil libraries for the Mono Visual Basic library,
aimed at the specifications / features of the Visual Basic 2012.
.
Mono is a platform for running and developing applications based on the
ECMA/ISO Standards. Mono is an open source effort led by Novell.
Mono provides a complete CLR (Common Language Runtime) including compiler and
runtime, which can produce and execute CIL (Common Intermediate Language)
bytecode (aka assemblies), and a class library.
The filename line tells you where to find the actual .deb file relative to the server root. Here, it would be https://download.mono-project.com/repo/ubuntu/pool/main/m/mono-basic/libmono-cecil-vb0.9-cil_4.7-0xamarin3+ubuntu1804b1_all.deb. That said, once you've found the thing you want in the package file, you can also just apt-get install mono-basic, and the package manager will take care of the rest.
