it'll be r134a that's only option these days.
For what it'll cost you you're better off getting it done properly though.
If it needs topping up that suggests its leaked or if it just needs regassing it needs exactly that and will need the correct amount of gas and oil refilling in it to ensure correct operation. You can't measure it out properly in a can.
Plus if you leak any of it to the atmosphere Team America World Police will turn up and abuse ya neighbours dog - it's serious business.
An aircon regas done properly is only about £40, a can of crap isn't much less.
Typically places will only charge if it regases too - if it leaks then you often get partial or no charge.
A proper regas hoovers out all the existing stuff in the system and then vacuum tests for leaks, so it'll tell you of everything but the smallest of leaks - which is useful to know.
If it leaks places usually offer a leak test option of pressurising with an inert and legal to leak out gas to help you too.