Both POF and Leitner-Wise are industry partners and have there own forums
POF ForumLeitner-Wise ForumPOF sells direct, plus through some vendors (i.e. DSA)
Leitner-Wise currently only does direct sales
ARES recently (3-4 weeks ago) announce the release of the GSR-35 but has since disappeared, I believe they will be doing direct sales if and when they bring them to market
HK direct sales to MIL/Gov. agencies only (No civi sales)
Prices is dependant on what type of set-up you are looking for. Best to contact each company with what you need for pricing.
We have four (soon to be adding a 6.8spc w/10.5" and a 6.8spc in a DMR format) LW uppers, I could not ask for a more relieable setup. But don't take my word for it as some might consider my POV as biased because of my connection with LW.
Read up and make an educated purchase. The POF, LW and HK systems have been accepted with great response from the actual end-users.