i dont think springs can leak, as theres nothing in there
it has to be the strut .. or the gas shock absorber.
Usually you'd want matching new things for the new shock, ie. the stress,usage, wear and tear of the old spring will not match the new shock per se, so wouldnt it be more ideal to replace the whole thing since the units are out anyway ? but at the end of the day, its the owners choice. Also yes, search for other places to get parts, theyre all generic. What you're paying for is the time and labour in the named businesses.
if you are handy with tools, you could actually even DIY, gives you a learning platform