Most of the networks offer "dual sim" phones, but be careful most are not true dual sims. They offer you the choice of inserting 2 sims or 1 sim and 1 sd card. In other words the sd card doesn't have its own slot. It uses on of the sim slots. If the specs don't specify that there is an sd slot, there probably isn't one. Also ask the salesperson to show you where the sims and sd cards go.
Almost all the "other brand" (i.e. not Samsung, LG, iPhone,...) phones are true dual sims. They are usually only available as out right purchases. They are cheaper than "branded" phones of the same spec. They will cost less than a phone as part of a contract. You own the phone from day one. On contract the phone belongs to the network until it is paid off. You thus have very little rights under the CPA when it goes wrong.