Among many others, there are regular services from Vienna, Prague and Budapest to Bratislava; and from Uzhhorod, Ukraine to the eastern Slovak town of Michalovce and from Krakow, Poland through Zakopane, Poland to Poprad.
Taking a bus from Prague to Bratislava is slower but cheaper than train if you buy a ticket in advance, e.g. at Student Agency, Slovak Lines, or using the common bus reservation system AMSBus.
Buses from Poland and Ukraine are the best option, they are faster and more frequent than trains.
From Budapest the travel is 4 hours, the bus stop for 5 minutes at GyÃ¶r and in a small restaurant in the road.