This article is from the Hanggliding and Paragliding FAQ, by Joao Geada with numerous contributions by others.
Ask around. Some pilots have old, unused supine harness they will sell
for a small amount. Sky Sports and Sunbird made very strong and
comfortable harnesses. Also several major harness manufacturers will
custom make one for you. Paraglider harnesses may be modifed by a
harness maker to create a quality supine harness.