if you got a slow puncture it is probably a leaking valve. You are better off taking your bike to a tyre place and getting them to fit and balance your tyres and get them to replace your valves front and rear too. Will prob cost you about £10-£15 per wheel on a ride in ride out. If you take them loose wheels they may not even ccharge you except for the valves and they are cheap. If you bring your own tyres in, even with loose wheels they may well charge you upto £30 per wheel as they won’t be making any money off tyres with you. Even with ebay prices once you add delivery you can end up spending a lot more by sourcing “cheaper” tyres online and then paying for them to be fitted.
Where abouts are you? Am sure someone local will be able to reccommend a decent place to go. I’d stay clear of places like Kwick-Fit, you’ll need a proper bike tyre specialist to make sure they fit them correctly.
Prob best not to go to a motorcycle dealership either as their tyres will be marked up and they charge lots on labour.