Instead of fiverr, try services from BHW. Right now it is the largest IM service marketplace that is still active and has decent quality services. Of course, be careful and choose smartly, read reviews, do your research. Since your clients likely do not want to get their site penalized, most likely you need to play it safe like you would with your own quality website. This means avoiding going with just the cheapest links, but going more for quality, diversification packages, quality web 2.0s.
From there you can start exploring PBN services that actually work and are high enough quality to be used for clients. Be careful many PBNs use spun content and might not be around in a few months. So proceed with caution.
You want to be able to show these links to clients. Contextual links should be readable quality.
A good way to go is guest post links. But very few do a real legit outreach campaigns. Most guest post services are just glorified PBN sites. To get good outreach links, you need to do it yourself. Services that do real outreach are usually in price range $100-150 per link.
If you go with fiverr services, you will most likely be scamming your clients with crap links that will get them penalized, so you will have to keep finding new clients, while old clients hate you for penalizing their sites.
Of course you could create high quality marketing/social media/SEO campaigns for your clients with fiverr services. But that would mean not just buying SEO/link-building services, but being creative and coming up with a plan to develop legit brand-building social media campaigns and finding an angle unique for each client.