Don't discount eBay completely, although there are a lot of vintage repop weapons and accessories listed on there it's probably still the best place to pick figures and vehicles up, ask the seller questions and if you are still unsure whether something is genuine or not move on. You can also post up questions on here if you need advice or a second opinion on something.
There are numerous Facebook groups but I'm not on Facebook so couldn't really say what they are like for sales or prices or give you any advice.
Of the 2 site links I think the first may sell through eBay anyway, the name seems to ring a bell, I'm not sure how old the toydreams site is but all the prices seem pretty low/out of date. C9 complete Princess Leia for £35.00, you wouldn't be able to buy her weapon for that now, C9 boxed Palitoy Empire Falcon for £110.00, C9 Blue Snaggletooth for £150.00 etc, so it's probably long defunct.
Hope this helps a bit.