If you mean this part AR41:
http://www.airgunspares.com/store/pr...Part-No.-AR41/
I can supply a brand new original for £12.50 by bank transfer, or £13.15 by Paypal, posted within the UK.
I have a small quantity of a fairly comprehensive selection of new Mk.3 spares including original mainsprings, but it would appear you've already sourced most of what you require.