There are some companies that have trial versions available, however, most provide money back guarantee, but you still need to pay. The logic behind that is for hosting companies to protect themselves from spammers and other illegal activities that the trial VPS usually is used for.
I agree with @hostens there are many VPS hosting company that allow free trial VPS for 30 days but you should check their services carefully or having pre-sale questions before buying so you will not need to request refund in the case VPS service didn't match your requirements.
Actually noone provides you free trial for 30 days, it's ridiculous. There are many that guarantee 30 days money back but with some ToS inside which you need to read it carefully. Also nowadays VPS are so low priced with 3€/month or even less, if you cannot afford it then don't use nor do anything regard this. Simple as that!
there are already vps providers the are like free. but don't want to recommend for trail offer. they maybe steal your money from paypal bank or credit card as a recurring trail offer.
i had bad experience one time like this similar case. i wanted to buy an ebook worth $89 but they offered it at $1. then quickly i purchased that ebook by card. after a week they again auto charge me another $90. my bank refused for reversal of payment so it was just like a bad experience of trail offers.