Cool. I am not sure where to buy such software but you could certainly make it/hire someone to do it. The complexity depends on the scale you want to achieve... targeting the same sites with multiple accounts will require multiple accounts(obv) but also payment details.. amazon caught onto this during the xbox/ps5 madness and limited addresses. Targeing multiple sites, each will have different apis, auth providers, checkout systems so custom programming required.
I considered this type of thing in the past but opted for a simpler approach. Instead of writing the scripts to complete check out I simply scraped the stock, when the items came in stock I posted to social media (twitter at the time) saying instock with my aff link.. eventually got me banned but I made a few quid. You could even do it without the scraper tbh and just lie
anyway with more effort any cloaking this is also a viable option..
Waffling now but people also sell services to notify people when items are in stock.. just another Spin on the same Idea.