Finding a Bra that fits one breast

Depending on your breast size, I could suggest the soft wire free ones from Best and Less.
They have zip up ones that I like best or crop top ones.
The zip up ones come in sports type material or soft cotton type material.
They are comfortable under my scar.
You need to get the ones with the removable padding as they have the little pockets for the breast form.
I know what you mean about the bra pulling sideways without the prosthesis to hold it in place, it also rides up under the arm on the side without a breast.
I am currently wearing a zip up soft one with the soft form prosthesis that came with the Care pack from BCNA after my mastectomy. 
The breast care nurse told me to use a small bag of rice in the bottom of the form to hold it down and in place, so I sewed a little polar fleece sleeve to hold the rice.
It is quite comfortable and not too hot, though it does move up a little at times (maybe I need more rice?)
The bras are only $10-15 so worth a try and come in fun colours.

Hi there I used the Kmart sports bras they are cheap as chips and super comfy. I have the prosthetics which I don’t like at all and am about to donate to Be Uplifted my local bc charity and they gave me a set of these really cute crocheted ones which are super soft and light and no rubbing or chaffing. They post them out for free so if you’d like some you can order them on the website. I had to wear a singlet under the prosthetics then pop them in the sports bras as with the warmer weather they were sweaty and horrible. I’ve had a double mastectomy 6 months apart so had a 6 month period with only one boob hope this helps. Margie x 

I have been feeling utterly incompetent on this subject, but now I just feel very fortunate. I wore the same bras post mastectomy that I wore pre mastectomy (with a soft pad) until I finally got fitted with a prosthesis about 12 months later. I never had a twinge from my scar, and simply assumed that was normal. The seroma was a nuisance, and was why fitting a prosthesis took so long, but was never sore. I'm impressed by everyone's inventiveness! 

The target post op bra is a little on the smaller cup size. I needed a full size up...so a 16 instead of a 14 so not too tight. It's  a compfy bra with all elastic nicely sewn inside. 

When I first started wearing a bra again after surgery I wore a really light singlet underneath it and it stopped some of the irritation over the scar.

Another tip if you end up with a sore scar is to try the silicon gel dressings.  I developed a keyloid and it was sore for a long time but the silicon gel dressings were really helpful at reducing the proud flesh and also making it feel protected and secure. 

Hi Pamela
i found bra fit feel much better after i removed the vertical  stays in the side seams, as these irritated the scar.Six years on still remove the stays  . Recommend soft sports bra as no stitched seams
All the best Bright

Target have their own brand of aah bras which are okay. 

Thanks @primek swapping instead of just throwing my bra back in the drawer sounds do-able. I just need to find some aah bras I'll look in Target & see what they have for post op wear & let you know!

I used to swap my bras throughout the day. I had aah bras...with pockets when it got too bad or just a soft post op bra like a Target one.  I was tender during chemo but its good now finished. Different bras have diffetent pressure so swapping helped.