On the basis that you will not find any governing bodies etc covering the purchase of houses in Bulgaria, you will be hard pushed to find a reputable agent other than getting personal recommendations - and I do mean personal recommendations. It would be very easy for an agent to recommend himself on this site saying that he has purchased a house there and is more than happy etc.
I would be very cautious of buying any property in a country where English is not the native tongue, where there is a lot of corruption (from what I have heard) and I don't understand the law.
In this country, buying a house is relatively simple - the question in buying a property in Bulgaria is who do you trust? Do you think the solicitor that you use will be governed by the law society? What happens if it all goes wrong?
I wouldn't buy in Bulgaria, but to be fair, I'm not that much of a gambler when it comes to my own money.
To answer you question, I know of two people who are involved with buying properties / developing land in Bulgaria and I wouldn't trust / recommend either.