What to do if your acoustic guitar has no plug in?

I began busking 2 months ago and have earned great money from nothing but my guitar and my voice, but I'm sick of having to find the perfect spot where my voice can be heard and I am sick of singing loudly. I have a roland cube but what do I use for my acoustic guitar? Do people use attachable mics for acoustic guitars? If so, can you recommend a budget friendly one?