S1L3NT X13
if you go to a store like best buy they can often tell you what works or you could chat online with them if there is no best buy in your area but any computer store could probably help you with modems however in the end your going to have to give as much info as you can to like your cable / satellite / isp company to have them figure out if what you have found is compatible
keep in mind that if you buy your own modem and anything goes wrong you will have to pay for your own maintenance on it and the cable / satellite / other isp provider will not assist you in fixing it so keep that in mind
its often cheaper to spend 5 / 8 /10 dollars a month to rent from your isp than it is to buy your own device because if it breaks down they have to give you a new one or fix it at no charge
No idea where you get they
have to, at least here in the US they could charge you $100 for a replacement modem if they wanted to, there's nothing technically preventing them from doing it.
Most simply generally give free modem replacements every 1-2 years because they feel it's good "Customer Relations", realistically though renting modems from the ISP when you factor they are at least on average (here in the US again that is), $10~ a month, you're effectively paying $120 in the first year for a modem that otherwise is low-grade and usually $50 if you'd buy it yourself.
Buy a good $100~ modem and it'll probably last you several years minimum
and still be cheaper than renting a modem for the first year alone.