You can use any math font package and its options (such as slantedgreek
). The isomath
package allows you to select any OML-encoded math font as one of your math alphabets, or you can use the LaTeX kernel’s \DeclareMathAlphabet
for this. The isomath
manual has a reasonably-comprehensive list of available fonts that support OML.
Additionally, mathastext
allows you to select any LGR-encoded 8-bit font as your Greek math alphabet, with any math version. Check out the LaTeX Font Catalogue.
It’s more work, but there are tools to re-encode any TrueType or OpenType font into legacy 8-bit TeX encodings usable with PDFTeX.
Examples
This MWE uses GFS Porson as the Greek math font. From roughly the 1920s to the 1960s, Porson was a common Greek companion font to Monotype Modern, on which DEK based Computer Modern. You’ll notice how similar the shapes are to the default TeX Greek symbols.
\documentclass{article}
\tracinglostchars=2
\usepackage[LGR,T1]{fontenc}
\usepackage{lmodern}
\usepackage[LGRgreek, italic]{mathastext}
\MTgreekfont{porson}
\Mathastext
% Can also declare \MTgreekfont for each \MTDeclareVersion
\pagestyle{empty}
\begin{document}
\[ (x, y, z) \to ( \rho, \phi, \theta ) \]
\end{document}

The following is an extremely-contrived example of defining an upright bold-sans-serif math version, to match the formatting of a section header, using GFS Complutum as the matching math font. (If you are ever stuck trying to do this in PDFTeX, you might try Arev, a heavy sans-serif font that comes in OML.)
\documentclass{article}
\tracinglostchars=2
\usepackage[LGR,T1]{fontenc}
\usepackage{lmodern}
\usepackage[LGRgreek,subdued]{mathastext}
\MTgreekfont{complutum}
\MTDeclareVersion[n]{sansbold}{T1}{cmbr}{bx}{n}
\pagestyle{empty}
\begin{document}
\Mathastextversion{sansbold}
\section{\normalfont\fontfamily{cmbr}\bfseries\selectfont%
From \(x\) and \(y\) to \(\theta\) and \(\phi\)%
}
\Mathastextversion{normal}
We can map \( (x,y,z) \to (\rho,\theta,\phi) \).
\end{document}
