As Alex says, you can load balance two or more lines using pretty much any reasonable pfsense / opnsense / openwrt router. I do it with two adsl lines, when you have multiple users it works pretty well. The easiest cheap way would be to use an old PC with a 4 port NIC.
The limitation compared to a bonded service is basically that only certain multi-threaded applications will be able to use all the band width of all connections at one time (e.g torrent) and a single connection is typically limited to the maximum speed of one connection. On our network the users are completely oblivious to the fact we have 2 connections and the advantage is that you don't need any kind of 'special' deal, I used to use two different ISP's for years and it worked great.
Just one warning, if my experience of the TP-Link load balancing routers is anything to go by (I had a TL-ER5120) , I would avoid. The firmware very flakey and unreliable and TP-Link were unable to fix it (they sent me beta firmware but eventually gave up and stopped supporting it less than a year after they stopped selling it)