If you want cheap and also the anticipation of not knowing if your order will turn up then I would recommend those online chinese sites we all know. To be honest I have actually had one or two decent purchases but you get what you pay for. Otherwise ASOS is usually the go to site for better quality at a lower price tag.

As heels are not a part of my civvy uniform, I prefer not to spend a huge amount of money on them. Places to try (sorry if you’re not in England)
Everything five pounds
ebay & Vinted (I’ve picked up some lovely brand-new ones are number of times for less than £10)

Indepedent shoe shops, charity shops (which are closer/easier than chain stores), online, also Amazon (I go for known brand names). I end up with a few good quality classics, which serve really well with a range of styles, clothing etc.

Justfab, they often have a promotion for new customers - I got my first pair of shoes for £10 instead of £35.
Ive also had some great bargains from eBay, if you search for strappy heels/stripper heels size 6 etc then use the search filters to show new items up for auction sorted by ending soonest you can sometimes get lucky - I got a brand new pair of FCUK strappy heels in the box for £8.50 with £3.90 postage. The price on the box was £110 and they'd never been worn. Had loads of bargains on eBay!

I really love Dorothy perkins for this one, they are my fav work shoes. And only cost me £30
Sorry they are stilettos but not platform ones