I was going to get round to making a thread about this at some point as i had the same question a while back.
There is some real tat about so know what brand etc you are after before ordering.
I've found AliExpress the cheapest if you don't mind waiting a while. (some can be within a week, longest I've waited was about a month). Plenty on ebay if you want it tomorrow. (its the same stuff as on Aliexpress, but more expensive, but you do have return options if there is an issue).
My favorite and cheapest pistol red dot is the Holy Warrior RMR. Its a pretty good quality replica. Better quality than the SRO I have and about the same as the RMRHD.
I've bought several from this seller (Outdoors Wonderland Store). Prices go up and down. Cheapest was just under $40 with some vouchers AliExpress seems to have on pretty much every month.
Smarter Shopping, Better Living! Aliexpress.com
www.aliexpress.com
Currently <£34 with a voucher.
All of the lights and optics below I've bought from AliExpress from June to Nov last year with prices I paid (some in usd some in gbp as sometimes the price is cheaper in GBP compared to the current USD exchange price)
Holy Warrior RMR $40
Not sure of brand, Streamlight TLR-1 $31
SpecPrecision RMR HD £77 (great quality)
Sotac X300U-B $49 (better fitment with the thumb screw than the non B version but more expensive)
Holy Warrior RMR
Not sure of brand, Streamlight TLR-7 - $48 (nice little compact light)
Sotac SRO - £37 (not as good build as the Holy Warriors)
Sotac X300U - $26 (cheap and very functional)
Holy Warrior S1, EXPS3 - £85
WADSN M600C (with Surefire logos) - $25
(i also purchased a Sotac M600C version a week prior and it came in a glossy black instead of matt black so it went in the bin. Previous Sotac version from a good while ago was fine).
Holy Warrior S2, XPS3 - $86
WADSN M300A (with Surefire logos) - £17
Not sure of brand, Vortex UH-1 Gen II - $66 (surprisingly good for the money)
Sotac M300b (with Surefire logos), from ebay £38 (bought a while back)
Happy to share links to all the sellers i picked up these up from if anyone wants them. (it can be a coin flip when choosing who to buy from).