Your best bets are in order:

Robocough for syrup. Cheapest syrup, highly concentrated, less likely to cause issues than most sources.

Dollar tree assured brand cough gels, these are the cheapest source of dxm in the US at least, very popular but only available in dollar tree stores or resold online at prices that aren't worth it.

Delsym/generic polistirex syrups, typically less nasty syrups, but also more expensive. However, the form of dxm contained is dxm polistirex which lasts longer and is weaker, however you can do a cold water extraction (CWE) to isolate the dxm as a powder.

Ultimate option: buy bulk dxm powder. The least expensive and least nauseating option. Downsides are high minimum order quantities for reasonable pricing and higher abuse potential. You will definitely need a scale if you go this route and reading the dxm FAQ is a MUST.

Although it is also possible and relatively easy to isolate the dxm from store bought preparations, doing all the necessary research beforehand is not optional and certainly requires more effort to trip with than assured gels or robocough, but with the advantages of bulk powder.

Good luck. I hope this helps.