Cannot vouch for any but am playing with the following:
BizBuySell, Business Brokers, Firm Exchange, Fit Small Business, BizQuest, Franchise Gator, Merger Place, Equire

I would not recommend focusing too much on inexpensive as if it's low barrier to entry for you, it's also low barrier to entry for everyone else! The old adage is true when it comes to 'free' and 'low cost' you get what you pay for! Referrals can be good, but they it's unpredictable and unscalable. I would recommend you focus on cold email to source off-market deals. This way the competition is significantly reduced and in some cases removed entirely and if you're able to really dial in your message, you can often times get a very high open and response rate, we currently get 80% open rate and 50% response rate on cold emails. Hope this helps.