Online courses could be the solution. While taking classes on your own wouldn't give you the career advancement or networking opportunities that come with traditional enrollment, classes on Coursera and other platforms can closely approximate the core curriculum of a top-notch MBA program.
Using the "curriculum snapshot" at the University of Pennsylvania's Wharton School of Business, we found close (or nearly exact!) free online matches for the six "fixed core" courses its MBAs are required to take:
Wharton requires: Foundations of Teamwork and Leadership
Online equivalent: Inspiring Leadership through Emotional Intelligence (Case Western Reserve University/Coursera)
Wharton requires: Marketing Management
Online equivalent: An Introduction to Marketing (Wharton/Coursera)
Wharton requires: Managing the Productive Core of the Firm: Quality and Productivity
Online equivalent: An Introduction to Operations Management (Wharton/Coursera)
Wharton requires: Microeconomic Foundations
Online equivalent: Introduction to Microeconomics (MIT/MIT OpenCourseWare)
Wharton requires: Advanced Topics in Managerial Economics
Online equivalent: Financial Analysis and Decision Making (TsinghuaX/edX, lectures in Chinese with English subtitles)
Wharton requires: Regression Analysis for Business
Online equivalent: Core Concepts in Data Analysis (Higher School of Economics/Coursera)
In addition to its fixed core requirements, Wharton offers "flexible core courses." The goal of these is to expand study in the core areas listed above (leadership, marketing, operations, microeconomics, economics, and statistics) while giving students more freedom to choose classes that suit their interests and goals. Wharton students are required to take one course in each of the required areas.
Here are some comparable online options for these:
Wharton offers: Financial Accounting
Online equivalent: An Introduction to Financial Accounting (Wharton/Coursera)
Wharton offers: Introduction to Corporate Finance
Online equivalent: An Introduction to Corporate Finance (Wharton/Coursera)
Wharton offers: Macroeconomics and the Global Economic Environment
Online equivalent: Economic Analysis - Macro (UC Berkeley/iTunesU)
Wharton offers: Responsibility in Global Management
Online equivalent: On Strategy: What Managers Can Learn from Great Philosophers (École Centrale Paris/Coursera)
Wharton offers: Managing the Emerging Enterprise
Online equivalent: How to Build a Startup (The Lean LaunchPad/Udacity)
Wharton offers: Dynamic Marketing Strategy
Online equivalent: Behavioral Economics in Action (University of TorontoX/edX)
Wharton offers: Innovation
Online equivalent: Innovation and Commercialization (MITx/edX)
Wharton offers: Advanced Persuasive Speaking
Online equivalent: Introduction to Public Speaking (University of Washington/Coursera)
If you get through all of that, you've completed the core requirements for a Wharton MBA. To earn a traditional degree, students need to take a total of 19 credit units (the core curriculum traditionally makes up nine of those, and the rest are electives). Most of these classes you can start taking now or else sign up to be notified when new sessions are open.
For other courses that could help round out your online MBA, we recommend checking out this summary of the best free online business courses.