The single necessary and sufficient condition for a business, says MIT’s Bill Aulet, is a paying customer. https://stripe.com/atlas or use a paypal button. Adapt this mentality. No investor will ever give you money. Make sales. Learn. Adjust. Repeat. Good Lucky! ;-)
I'm thinking more about support than money. I can run it for some time on my own for sure... But registering the company, talking about marketing, helping with sales strategy all these things.. I definitely need them more than money.
But I would be hesitant to pay (much) to join one, there seem to be a lot of faux-accelerators out there preying on small companies. Ideal would be one that accepts people based on merits. Paying a few hundred dollars a month is reasonable, but giving up 10% of your company is not. That mentorship just isn't that expensive to provide to you.
The founder of a startup must learn marketing and sales. This is unavoidable. You're the only one that deeply understands the product, and hopefully the market segment. If you don't understand the market segment, you must learn yourself. No marketing expert or sales firm will do as well as you yourself will.
Once you've figured out how to sell it, which a mentor can help with but not substitute for, then you can train a sales person to "replicate" it. Once you have some example sales, you can define your "customer profile" which any sales person will need to have in order to know who to target in outbound sales to develop leads. Then the sales person would mostly be working on qualifying those leads to see if they truly are a good fit to sell to, and ultimately mostly just following up with them to move the process forward.
Basically, assume that you are the lead sales person. That has to be your role now, there's no other option. Find the training you need, even if it's just an online crash course in sales. Don't hire a sales person until you feel confident you could clearly define not just that you need sales or vague ideas about who your target market is, but what characteristics of a customer to look for are.
Sales isn't really all that hard. But it's also not easy, it requires a lot of willingness to hear "no" and not feel bad about it or move heaven and earth to get them to say yes. They won't. You need to be methodical and persistent, not take anything personally ever, and try to eliminate leads from your list of possibilities as fast as possible if they don't truly look like they'll turn into a sale. That's the only way to do it efficiently.
(I am talking about enterprise sales, FYI. I don't know much about consumer sales.)
EDIT: I noticed elsewhere that your customers appear to be companies. In that case, I believe my advice above applies well. You're likely to have a modest number of customers, who are hopefully paying a fairly high amount per customer. This is the mode in which enterprise sales work best, since it requires a bit heavier a touch (i.e. calling specific people at a potential customer to followup) than consumer sales.