Philips Norelco shaving heads for series 5000/6000 Model SH50/52 $19.49 with free shipping.
https://www.usa.philips
Get an extra 15% off if you signup for their newsletter, bringing them down to $16.52
These are $30+ on most sites. Be aware of the knockoffs on Amazon/Walmart sold by 3rd parties they look the same but are not very good.
Fits S5000 (S5xxx), excl. S55xx
Fits AquaTouch (AT8xx, AT7xx, S5xxx)
Fits S6000 (S6xxx)
Fits PowerTouch (PT8xx, PT7xx)