The best and probably the safest internal compressor is made by OTC, but it is pricey at $200. Other ones go for $40 to $50. Try to find a USA made compressor. (Mine is an old KD Tools, USA). There are a lot of generic name ones available and I would guess most are made offshore, and the quality can be hit or miss. Just do a search for spring compressors and read the customer reviews if any. Toolsource.com has a USA made compressor for $47. I would say any compressor in the $40+ price range would suit your needs. Ive seen some listed for as little as $17 and would stay away from those, though maybe some forum members had good luck with them.