Are you just looking for an inexpensive receiver and you are going to drill the holes for the underfolder, I believe that is what most recommend otherwise the underfolder receiver block may be mislocated causing the receiver cover to be to far back on some builds? If so, Inrange is using the
NoDakSpud economy receiver for the SA85 full stock rifles so he must believe they are quality for an inexpensive receiver. They don't have the step down top rail and the trigger pin hole appears to be elongated like in the Generation I Ewbanks receivers.
DPHarms used to carry Ewbanks receivers for underfolders but haven't seen him carrying them in awhile.
For a quality receiver you can get the Global Trades/Armory USA receiver, I thought they had them drilled for underfolders but their outdated website doesn't show them that way, Vector uses these for their builds. You could ask Jimmy Streetman on the Global Trades forum at gunsnet or warrifles.com. Usually a backlog.
Best bet is most likely a receiver without trunnion holes drilled so it can be drilled to match up right. Global Trades.