Charissa,
Since the posts above have nicely defined "solo ads", I'll share some recommendations here regarding direct marketing tactics...
1. If you're in MMO (Make Money Online) / IM (Internet Marketing) / Get Rich Quick niches, then I don't recommend purchasing solo ad spots from providers in these niches. Based on our experience with our clients who operate in these niches -- The best results in terms of RoI is to drive targeted social traffic and organic search traffic to your incentivized opt-in offer, in order to build your mailing list, through a balanced combination of paid campaigns like advertising in credible networks and online places that can effectively reach your target audiences, and through free campaigns like actively participating and initiating relevant discussions with value in directly and laterally relevant online places that are heavily trafficked by specific groups of people most likely interested in the value of those incentivized opt-in offers...
2. If you operate in other niches aside from those that I mentioned above, then I suggest making a list of credible, wildly popular online places in directly and laterally relevant niches. Then, prune this list into online places that are most likely heavily trafficked and actively participated in by your target audience. Next, prune your shortlist further down to online places that have opt-in offers and mailing list subscription sections. Then, directly contact the admins / Webmasters / owners of those online places, in order to negotiate mutually beneficial mailing list ad placement deals with them (or even paid ad banner / text link ad / advertorial placement deals and guest posting or content syndication arrangements with them). Based on the results of such campaigns that we execute for our clients -- This generates better results in terms of RoI, compared to going with providers that actively promote solo ad placement offers in niches outside MMO / IM / Get Rich Quick...
3. If you have a fully optimized sales funnel (incentivized opt-in offer, opt-in copy, top quality and timely onsite content, non-promotional follow-up emails and promotional follow-up emails with compelling recommendations of your own products or your affiliate products), then I highly recommend going with reputable list brokers that are authorized official members of fully licensed Direct Marketing Associations (DMAs) in accredited national trade organizations of the countries where your target audience is based. This often requires significant marketing investment funds, but based on our experience with our corporate clients for our inbound call generation campaigns in our telesales contracts with these corporate clients of ours -- This generates the best results in terms of RoI, compared to the other things I mentioned above, HANDS DOWN...
That's it for now. Cheers!