Cheapest but toughest way is SBA [0] Financing through traditional banks. You get the lowest interest rates (usually Prime + 1-2% extra) with upto 10 years of payment term. The problem is that it can take 2-3 months with tons of documentation and a personal guarantee if you are small. I have some experience with this for our own SAAS so happy to share more if needed.

I wouldn't rule SBA backed loans out because the more expensive options may give you capital faster but you pay a lot more premium. And when I say "lot more", I mean "a hell of a lot more premium"..

[0] https://www.sba.gov/funding-programs/loans