It depends on your build as to what might suit you. I spent 2 years working at Next and can wholeheartedly recommend their suits. Cheap, good quality and a wide variety of sizes, especially for smaller guys (goes down to a 26 waist and a 32 jacket which was a godsend for me) and if you go into the store they tend to be very on top of service, especially during prom time because suits bring in all the revenue so go have a chat with an adviser.
You're saying they look boxy, I mean suits are meant to make you look very square so what kind of thing are you looking for. For example if you're looking at tailored fit and finding it's too straight and 'boxy' then I'd say try a slim fit.
A couple of other cheaper shops that I can recommend suits for is river island (goes down to extra short trouser leg rather than just short which can be helpful for some guys), topman and if you're really on a budget then actually primark suits aren't the worst but the price tag means the fit and size may be a bit off.
At the end of the day you have to go and try things on to see what works, different brands fit differently even if they're supposedly the same fit and size so try a couple of places. Asos can also be good. If you order a few online, try them on at home and then it's free delivery to send back what you don't want.
Sometimes hiring can cost just as much as buying one outright (and a suit will always come in handy in the future), but Moss Bross has a hire brochure online that you're able to view and they're a very good shop for suits too.
