A quick internet search came up with:
Dawkes https://www.dawkes.co.uk/spare-parts/trumpet (with whom you already have experience)
Thomann https://www.thomann.de/gb/replacement_parts_for_brass_wind_instruments.html
East Coast Instruments https://www.eastcoastinstruments.com/shop/index.php?route=product/search&search=trumpet valve springs (with whom I have no experience)
Although the cost of custom springs may be high, you might contact https://www.airedalesprings.co.uk/springs/compression-springs/ .
Getting in touch with these companies may result in success or at least point you toward other suppliers.
Clipping springs, while lowering the height, increases spring rate, rather than reducing it http://kbdelta.com/blog/spring-rate-what-and-how-calculated/ .