I have the answers you seek ... My other half has been collecting sisters of battle for about a year and she loathes painting metal sisters so has been doing her best to get plastic sisters.
As a meticulous painter who wont assemble until painted her pace of collecting can be best be described as glacial so only has has about 1000 points and only 2 units of sisters finished ... way too many nice shiny elites isntead * so we have been playing kill team , We use counts as Battle sisters as they can basically take all the same weapon options as tactical marines , A Seraphim as a Reiver Leader with power sword and heavy bolt pistol
Now as for cheap sisters options " The Good option
https://wargameexclusive.com/shop/battle-sisters/emperor-sisters-squad-upgrade-5u/
These are amazing the quality is outstanding and you get 5 for 20 Euro the downside is delivery time ... these are coming from poland and will take time to ship , we have their Seraphim and some
HQ choices and these models are drop dead amazing , you can get special weapons as well but they are a bit expensive .
Here's the reiver Squad leader sister seraphim
https://drive.google.com/file/d/1YQgPf-814B9A-mAvGqYBYxSfxUYZl2fU/view?usp=sharing
Off brand options
These are good scale wise but a definately female marine style rather than sisters of battle.
http://shop.prodosgames.com/space-crusade/265-armoured-crusaders.html
The Legit
GW option is to buy sisters of silence you get a box of 5 for around £20 GBP and then source yourself some Skitari Vanguard heads for the sabbat helm's or buy a set of female faces from here
http://www.statuesqueminiatures.co.uk/p/9239907/sma352-heroic-scale-female-heads-narrow-2.html
The size you are after is heroic narrow, which suit the sisters of silence perfectly
Finally you can try grishnak models he is a russian sculpter who makes sisters of battle stuff , i have heard his quality is very very good but i've not seen these in person ... and apparently shipping times can be a bit hit and miss , he only deals direct via email ...
https://steemit.com/games/@jacobtothe/grishnak-models-war-maidens-review