I'll heartily second getting yourself a little puck of Master's, although there are a number of viable alternatives, when it comes to brush cleaners/restorers/preservatives. Equally, if not
more, important is how you use and clean your brushes. Rinse frequently during painting, avoid excess pressure, use cleaners as a preventative (as opposed to a repair), etc.
Being on the other side of an ocean, though, I can't recommend local stores/brands for quick and easy acquisition of replacements. Normally, I'd recommend a cheap brand of a similar material type (i.e. natural, Taklon, etc.) that behaves more or less like a quality brush you might use, then tell you to practice use and maintenance.
As an exception in your
specific case, being in the
UK, I'll suggest Rosemary & Co. Amazing brushes (although not the
very best, they're certainly up there) and surprisingly cheap, especially considering the quality. Definitely above craft/gaming store fare in use and resilience, while costing the same or less (significantly, in some cases). They're my go-to brushes... and I had to pay transatlantic shipping to get a sampler.