5x7 enlargers have been made by any number of manufacturers. Some of the most common names are Durst, Elwood, Omega, although others certainly exist. My only experience is with the Durst which is a wonderful machine and came in a number of configurations. Check Ebay, as usual. They show up with some frequency. It's a buyer's market. Prices may range between $200 and $2000, but generally fall below $1000. Condition and location are the usual deal-breakers. Whatever you do decide to get, make sure it's complete, i.e., with negative carrier, condensers, baseboard, etc. Spares are available, but you can spend as much on a negative carrier as on the whole enlarger these days.
I have a Durst 138 and a Durst 138 S models (both 5X7) that are surplus in my possession for sale. If you are interested let me know by PM. These are both used and in excellent condition. The difference between the two is that the S model does have foot assisted baseboard movement.
i use an omega e4 - it has the ROUND lightsource
there is also the e3 - it has a SQUARE lightsource.
while they are similar enlargers, the e3 has a shuttered lens because the lightsource must remain ON all the time. the e4 uses a circline fluorescent bulb. they came as cold light enlargers ( omegalite ), but sometimes you can get lucky and find a e5 condensor head for them.
there is a company in canada http://www.khbphotografix.com/ - is an omega distributer & can fabricate parts & repair if needed. for example, i needed a new negative carrier - machined metal, tiny parts & glass. they were going to make me one NEW for $40 us.